View Full Version : British Bases in Cyprus and Signals Intelligence

Magda Hassan
08-20-2012, 01:16 PM
British Bases in Cyprus and Signals Intelligence

Giorgos Georgiou

The Embassy does not believe that the loss of Cyprus-owned physical
infrastructure, nor the interruption of key resource exports from the island,
would immediately affect the security, national economic security, and/or
public health or safety of the United States. A Connecticut-sized
Mediterranean island some 5000 miles from the East Coast of the United
States, Cyprus simply is too small, too distant, and too lacking in natural
resources to affect U.S. interests in that fashion. […] Under the terms of the
1960 independence treaties, Great Britain was allowed to retain two
"Sovereign Base Areas" (SBAs) and several isolated sites scattered
throughout Cyprus, such as the RAF radar dome on Mt. Olympus, the
island's highest point, and various antenna arrays in Ayios Nikolaos, near
Famagusta. Via varied formal agreements and informal arrangements, the
United States enjoys some access to and benefits from these UK facilities.
Unlike the Cyprus-owned infrastructure noted above, the damage or
complete loss of SBA-housed facilities would pose a threat to our national
security interests in the eastern Mediterranean. 1

This arrogant but perhaps realistic assessment by the US Ambassador in
Nicosia sets the stage for what follows in this brief report. Comprising 254 sq
km or approximately 3% of Cyprus territory, the so-called Sovereign Base
Areas are probably the most important Western Intelligence listening post in
the Eastern Mediterranean, monitoring communications in the Middle East,
North Africa, the Balkans, Russia and beyond. Cyprus has also been a
staging point for various UK-US Intelligence and military operations, one of
the earliest being the overthrow of Iran’s Prime Minister Mohammad
Mossadegh in 1953 in the now infamous Operation AJAX. 2 Until very recently
RAF Akrotiri was actively involved in the reconnaissance and air support of
the war in Libya, as part of Operation Ellamy.

It was not until June 1947 that the full potential of Cyprus as a listening post
started to be exploited, with the transfer of British personnel and equipment
from Palestine and the creation of the Ayios Nikolaos Station near
Famagusta, to house the 2 Wireless Regiment, later renamed as the 9 Signal
Regiment. Soon, another British Signals Intelligence (SIGINT) station was
moved from Iraq to the village of Pergamos, near Ayios Nikolaos, were it
operated until September 1968. Up to one thousand British radio operators
and technicians were stationed in the two sites during those first years, living

Confidential diplomatic cable sent by US Ambassador in Nicosia Ronald L. Schlicher to the State
Department on 29 January 2008, published by WikiLeaks on 1 September 2011 (Reference ID
08NICOSIA70). See http://www.cablegatesearch.net.
See Politis daily newspaper, 10 October 2000, p. 16.

in tents, surrounded by mud and exposed to the elements. 3 Through the years
the Ayios Nikolaos Station, in 1999 renamed as Joint Service Signals Unit
(Cyprus), became one of the largest and most important SIGINT and ELINT
(Electronic Intelligence) sites of the British Government Communications
Headquarters (GCHQ), second only to RAF Menwith Hill in Yorkshire.

Cooperation between GCHQ and its US counterpart, the National Security
Agency (NSA), in Cyprus was close from the beginning, albeit sometimes
tense, based on the UKUSA Security Agreement. 4 Ayios Nikolaos Station
collected SIGINT and ELINT during the 1948 Arab-Israeli War, the 1956 Suez
Crisis, the EOKA 1955-1959 Cypriot liberation struggle against British
colonialism, 5 the clashes between Greek- and Turkish Cypriots in 1963, the
1967 Six-Day War and the 1973 Yom Kippur War, to name just a few of the
older and seemingly never-ending conflicts in this volatile part of the world.
The final Intelligence product was of huge importance to the decision makers
in London and Washington, as it allowed them to plan and shape their policies
with the immense advantage of near real-time flow of accurate information
from the political arena as well as the battlefield.

Fine details of the way Ayios Nikolaos operated in the 1960s were revealed to
the Scottish investigative journalist Duncan Campbell by Corporal John Berry
on 18 February1977. Berry had served at Ayios Nikolaos from 1966 until 1970
and gave Campbell an extensive account of his work there:

We were responsible for Iraqi, Egyptian, Greek Cypriot and Turkish Cypriot
illicit communications. We also intercepted some Israeli, although I was
never really sure who was basically responsible for Israeli interceptions,
whether it was us or the Americans. There was certainly a task – there was
a bank of sets in the intercept room that picked up Israeli communications.
But we also got stuff fed back from NSA that they picked up as well. We
had an American at 9 Signals. A guy called Dave Mallory who was NSA. I
was involved with Iraqi intercept. And we had specific tasks, which were
allocated to us from GCHQ. We had to intercept Iraqi Diplomatic, Air Force
and Military traffic. Once you’ve identified a net, as an Iraqi Army net, it’s
fairly elementary, especially the Arab countries, to keep track of it. I mean
the Russians employ very sophisticated techniques with changes of
frequency and everything else. The Arabs employ much less sophisticated
techniques and it’s fairly easy to break their frequency rotas and times of
new schedules, the times at which they transmit. Most of the Intelligence

For a more detailed review of the creation of the Ayios Nikolaos Station see the first part of my TV
documentary Το Νησί των Κατασκόπων (The Island of Spies) first broadcast by the CyBC on 24 March
2005 and available online at http://video.google.com/videoplay?docid=620430361579534374. See
also Richard J. Aldrich, GCHQ, HarperPress, 2010, pp. 161-163, and Brendan O’Malley and Ian Craig,
The Cyprus Conspiracy, I.B. Tauris, 1999, pp. 79-82.
See Nicky Hager’s investigative masterpiece Secret Power, Craig Potton Publishing, 1996, and
O’Malley and Craig, op. cit., pp. 83 and 125.
See in general Peter Wright, Spycatcher, Viking, 1987, pp. 153-159.

that we derived from Arab communications was concerned with
whereabouts of units and the buildup subsequent to the Middle East War.
We were very anxious to have information about the sort of Iraqi presence
in Jordan in the 18 months following the June war in ’67. […] I certainly saw
lots of other NATO countries intercept. Which was – well I mean a lot of it
was casual, because the operators would be tuned in to a particular
frequency, would pick up some Italian Diplomatic traffic or some French.
[…] It was possible to get quite a lot of information about what the state of
arms in the Turkish Cypriot fighting units was. Quite a lot of the
communications were concerned with – and also interestingly – with illegal
acquisition of British arms, because there was quite a leak at one point.
And various people were court-martialled selling British Army rifles to
Turkish Cypriots. 6

The range and scope of British and American SIGINT operations in Cyprus
during the first decades of the Cold War become more evident after a very
recent revelation concerning the mysterious circumstances surrounding the
death of the UN Secretary General Dag Hammarskjold in a supposed plane
accident on 18 September 1961 in Northern Rhodesia - now Zambia. An
American Intelligence officer stationed at the NSA Naval Security Group
Command installation at Yerolakkos, 7 west of Nicosia, recounts how he
intercepted the transmission of the pilot of another plane in the area of the
subsequent crash, closing in on the DC6 with Hammarskjold on board, firing
his plane’s guns and then shouting "I've hit it". 8 The American officer was thus
able to monitor clearly the apparent attack that took place 5,340 km away.

A few years later the NSA station at Yerolakkos focused on the Palestinian
Liberation Organization, Yasser Arafat’s Fatah party and the Black September
Organization, sometimes with deadly results. An NSA Arabic linguist
responsible for the PLO Desk at Yerolakkos revealed that “NSA would send
intercepts detailing the movements of suspected terrorists to the CIA, which
would in turn pass them on to Israeli intelligence — and some guy was dead.
We’d get a message that so and so was coming through the Athens Airport

To the best of my knowledge this is the first time that even a part of Berry’s original account is
published, as it is still considered Top Secret by Her Majesty's Government. The British Security
Service, more widely known as MI5, had been monitoring Berry’s account to Campbell and
immediately arrested both. For their subsequent prosecution, the ABC Trial, see Geoffrey Robertson,
The Justice Game, Vintage, 1999, pp. 104-134.
This NSA station operated from June 1957 until 23 April 1974. Other US monitoring sites in Cyprus
included Mia Milia to the northeast of Nicosia and the US embassy. A CIA Foreign Broadcast
Information Service (FBIS) station, collecting, translating and analyzing Intelligence from open sources
in countries of the Iron Curtain was operating at Karavas. See O’Malley and Craig, op. cit., pp. 82-84.
FBIS Karavas was established in 1949. At the time of its closure on the day of the Turkish invasion on
20 July 1974 it had 85 employees, including 13 Americans. See document titled Eyewitness Reports
From The FBIS Mediterranean Bureau 20-23 July 1974, dated 18 September 1974, describing the
employees’ escape to safety.
Susan Williams, Who Killed Hammarskjold?, C Hurst & Co (Publishers) Ltd, 2011, pp. 139-150.

and then you’d read in the Jerusalem Post that some poor sucker had been
killed in an airport." 9

British and American SIGINT installations in Cyprus continued their work
throughout Turkey’s invasion in 1974 and intercepted the very message by
which the military junta in Nicosia reported to its Athens headquarters that the
coup d’état against Archbishop Makarios had begun. 10 Other examples of
Ayios Nikolaos activities included monitoring the 1980-1988 Iran-Iraq war and
the Soviet naval presence in the Eastern Mediterranean. Monitoring Soviet
ballistic missile tests in mainland USSR was possible since around 1964 by
use of a US RCA company designed over-the-horizon (OTH) backscatter
radar (codenamed COBRA SHOE) in the Akrotiri SBA. 11

Apart from various High Frequency (HF) antennae used for intercepting
mainly military and diplomatic signals from around the world, Ayios Nikolaos
also houses a HF Direction Finder (DF) that enables its operators to
triangulate, pinpoint and analyze transmissions within a range of thousands of
kilometers, monitoring bearings of transmitters and signal strength, thus
enabling the construction of a virtual electronic map of troop movements and
similar activities. Next to the HF DF, several satellite antennae form part of the
UKUSA ECHELON system, 12 automatically intercepting fax, emails, Internet,
mobile phones, telephones etc from a multitude of geostationary
communications satellites. However, the SIGINT importance of ECHELON
has been in drastic decline during the past decade due to the wide use of
fiber-optic communications. 13

Twenty kilometers east of Ayios Nikolaos, Cape Greco used to be a NATO
early warning communications site, part of the ACE HIGH 14 network until the
late 1980s. It ran from Norway through the UK and France, Italy, Greece and
Turkey, where the Incirlik Air Base was linked with Cape Greco via
troposcatter antennae. Cape Greco was in turn connected via microwave link
to the RAF radar on top of Mount Olympus, providing NATO with live
coverage of the airspace in the area. Armed Avro Vulcan strategic bombers
were stationed at RAF Akrotiri as part of Britain’s nuclear deterrent from 1969

See James Bamford’s supplemental notes with updates published online for Body of Secrets, Anchor
Books, 2001, at http://www.randomhouse.com/features/bamford/notes.html#page166.
O’Malley and Craig, op. cit., p. 168.
According to US government declassified documents. There are indications that this OTHR was
moved to Ayios Nikolaos in the years following 1974. Whatever the case, the design and orientation
of a specific antenna to the southeast of the Ayios Nikolaos station suggest it is an OTHR covering the
former USSR. See the third part of my TV documentary Το Νησί των Κατασκόπων (The Island of Spies),
op. cit. Another OTHR, code named AN/FSQ-76 (over-the-horizon forwardscatter receiver) was
located in Cyprus, probably in the 1950s and 1960s, to detect missile launches in the USSR.
See Hager, op. cit. and second part of my TV documentary Το Νησί των Κατασκόπων (The Island of
Spies), op. cit.
James Bamford, The Shadow Factory, Doubleday, 2008, pp. 161-163.
See http://www.subbrit.org.uk/rsg/features/ace_high.

to 1975, and were ready to retaliate in case of conflict with the Warsaw Pact,
if the order was given via the ACE HIGH network. In 1994 the Cape Greco
installation was destroyed with explosives by the British army before the site
was returned to the Republic of Cyprus to become a National Guard camp.

Since 1970 Cape Greco has also been home to a powerful HF transmitter for
Radio France. Since 2002 Voice of America also uses the 2MW transmitter,
whereas British propaganda is broadcast by the British East Mediterranean
Relay Station in Zygi and its apparent sister station just west of the Lady’s
Mile beach in Limassol, with both transmitting the BBC World Service in
several languages. All stations cover much of the Middle East and North

The RAF radar on Mount Olympus continues to monitor civil and military air
traffic over and around Cyprus, Syria, Lebanon, Israel and parts of Egypt and
Turkey. The Troodos Station, Golf Section, formed in 1878 near the Troodos
Square, is the oldest remaining British site in Cyprus. Being a detachment of
the JSSU, it provides military planners and commanders with invaluable
ELINT collected with a variety of sensors, looking for weaknesses in the
defense networks of neighboring countries and detecting ships and aircraft by
their radar and other electromagnetic radiation. The addition of the Starbrook
system in 2006 allows Britain to scan the skies for satellites and other objects
in space as small as 1,5m. 15

The Episkopi Satellite Ground Section is also part of the JSSU and its primary
role, according to the MoD, is to provide secure satellite communications off
Cyprus, probably via the British Skynet satellite system, bridging the gap with
the UK. Recently two of the three radomes of the station were removed;
however security around the remaining radome and the nearby HF DF
continues to be very high.

For many years the Akrotiri SBA was the site of a powerful shortwave
transmitter, the Lincolnshire Poacher, a “Numbers Station” used by the Secret
Intelligence Service (MI6) to transmit encrypted one-time pad instructions to
undercover British agents in unfriendly regimes. The station went silent in July
2008, perhaps indicating that British spies have found easier and more
efficient ways to communicate with their Legoland headquarters on the bank
of the river Thames.

Next to the Akrotiri salt lake lies a huge curtain antenna, code-named PLUTO,
which is a 100m tall, 200m wide, 1 MW OTH radar, detecting airplanes and
missiles in countries as far as Afghanistan, Kazakhstan and parts of the
Russian Federation. The vast receiving array of this OTHR lies in three
rectangular structures at Ayios Nikolaos.

See the European Space Agency’s Bulletin 133, February 2008.

RAF Akrotiri is the biggest British air base outside the UK. Apart from its role
as a staging post for military planes and troops for a wide range of operations,
since the 1960s it has also been the base of American U-2 spy planes, flying
as high as 20 km as far as Afghanistan, monitoring communications and
collecting high resolution photos of territories being overflown. Recently the U-
2s in Akrotiri participated in the enforcement of the no-fly zone over Libya.
More controversial, however, is their use over Turkey and Lebanon, which in
2008 caused a diplomatic incident with the British government.

As revealed by WikiLeaks diplomatic cables, the U-2s were being used by the
US to fly missions codenamed Cedar Sweep over Hezbollah locations in
Lebanon, after which the Americans passed the Intelligence to the Lebanese
authorities to help them track down Hezbollah militants. Likewise, the U-2s
gathered Intelligence over Turkey and northern Iraq and the information was
secretly supplied to the Turkish authorities in an operation codenamed
Highland Warrior. The UK protested that "in both cases, intelligence product is
intended to be passed to third-party governments". What the UK actually
feared was the possibility of being complicit in unlawful acts, or the Cyprus
government finding out. 16

It would be an omission in the context of this report not to refer briefly to some
more recent US SIGINT operations in Cyprus. One was NSA’s alleged
collaboration with Crypto AG, the Swiss cryptographic and secure
communications supplier trusted by many governments worldwide. According
to various sources, NSA agents convinced Crypto AG to install backdoors on
its equipment sold around the world, enabling the Americans to access the
encrypted traffic produced by the machines used by foreign governments.
From 1972 until 1992 Cyprus had been using 13 Crypto AG teleprinters
linking the Ministry of Foreign Affairs in Nicosia with Cyprus diplomatic
missions abroad. When the Cyprus government was informed of the possibly
compromised machines in 1992, their use was immediately discontinued.
However, if the teleprinters were indeed rigged, it would mean that for twenty
years the Americans had unimpeded access to virtually all the diplomatic
correspondence of Cyprus. 17

Another operation of the NSA in Cyprus, still ongoing, is enjoying diplomatic
immunity. The roof of the US embassy in Nicosia is covered with various
kinds of SIGINT antennae, installed by the secretive Special Collection

See The Guardian story titled UK overruled on Lebanon spy flights from Cyprus, WikiLeaks cables
reveal at http://www.guardian.co.uk/world/2010/dec/01/wikileaks-cables-cyprus-rendition-torture
CLEARANCES at http://www.cablegatesearch.net.
See my TV report for the CyBC, broadcast 15 July 1999, partly reproduced in the Greek daily
Eleftherotypia on 17 July 1999, p. 3. For more information on this extraordinary story see also

Service 18 and able to intercept nearly the full spectrum of frequencies. Mike
Frost, a veteran of the Communications Security Establishment, the Canadian
counterpart of the NSA, was trained by the SCS in Maryland on how to install
similar systems in US and Canadian embassies around the globe and
amassed 34 years of experience in SIGINT. In November 2000, together with
Duncan Campbell, Frost prepared a detailed analysis of several photos of the
US embassy in Nicosia, concluding inter alia that

At least part of the top floor of the US Embassy, Cyprus, is being used as
an Operations Room to conduct a massive SIGINT collection operation.
This operation consists of at least eight whip antennae of varying length, at
least eight semi-directional Yagi antennae, two possible long wire HF
antennae and at least five antenna boxes used to conceal an assortment of
directional antennae. From the number and types of antennae and boxes
visible it is thought that this is a 24/7 operation and covers most if not all of
the radio spectrum. Below is a partial list of signals a site such as this
would be able to listen to and copy: cellular telephones, nationwide and two
way pagers, Internet communications, analogue and digital cordless
telephones, radio frequency wireless modems, satellite telephone up/down
links, terrestrial and satellite microwave communications, credit card and
ATM transactions, HF radio communications and Tempest Radiation. 19

It is clear from Frost’s analysis that the US embassy in Cyprus has sought and
indeed has managed to complement to a great extent the SIGINT obtained
and shared by the British on the island. This applies also to open source
Intelligence gathering. A year after the closure of the FBIS station at Karavas
in 1974, the US government obtained permission by the Cypriot Council of
Ministers to reopen the station in Nicosia, where it operated as part of the US
embassy. In 1985, FBIS was relocated to its present site at Archangelos, 20
having been absorbed by the Open Source Center in 2005, according to the
recommendation of the 9/11 Commission. The station employs automatic
speech recognition and translation and several other technologies to gather
Intelligence from a variety of open sources including the Internet, public
databases, the Press, radio, television, video, geospatial data, photos and
commercial imagery. In total, the OSC collects open source Intelligence from
more than 160 countries in more than 80 languages. 21

See Bamford, The Shadow Factory, op. cit., pp. 213-214 and also Bamford, Body of Secrets, op. cit.,
pp. 477 and 479-480.
Based on Frost’s assessment I prepared an explosive story for the Politis daily newspaper, the
owner of which eventually refused to publish it without even covering my agreed expenses. In the
end it was published in two parts on 20 and 21 January 2001 on the front page of the daily Haravgi
newspaper, which predictably excluded from my story Frost’s SIGINT analysis of the Russian Embassy
in Nicosia.
Cypriot Government Spokesman statement, 11 March 1987.
See OSC website at https://www.opensource.gov.

Despite being a Connecticut-sized Mediterranean island, too small and too
distant from the US, for the better part of a century Cyprus has been and
continues to be vital for Western strategic interests. During the Cold War,
Intelligence gathered in Cyprus helped maintain the fragile balance with the
Soviet Bloc, avoiding a destructive global nuclear war and projecting UK, US
and NATO power in the Middle East and beyond. Unfortunately, the Cyprus
state failed utterly to participate in this regional Great Game even as an active
spectator. Instead of gain, there was great loss.

Since the beginning of the 21st century new transnational challenges have
emerged. International terrorism and militant forms of radicalization, weapons
proliferation, international organized crime, narcotics, human trafficking and
the geo-political implications of climate change are now at the top of the
agenda for policymakers worldwide. 22 It remains to be seen if Cyprus will find
a way to get even a small share of the profits in the international trade of

Roger Z. George, Meeting 21 Century Transnational Challenges: Building a Global Intelligence
Paradigm, Center for the Study of Intelligence, Studies in Intelligence, vol. 151, no. 3, 10 September
https://docs.google.com/viewer?a=v&q=cache:fvurXf88uYoJ:cryptome.org/2012/01/0060.pdf+British+intelligence+monitoring+Syria+fro m+Cyprus&hl=en&gl=uk&pid=bl&srcid=ADGEESh5QpIpW3Hkaz-NfhOMfch9DLgkeBfw8wmvrX6gcMwiYjO3uQfnUXzxDwDICXFeZ 5DYDiKdYoGbN1SEttZlyvnU9TL1zjZFTzmh2GnHW_uZT4iVSdK s6CU4rOh3vPtWYU9QPNs2&sig=AHIEtbRGd6JUVWgUKDbDLiycWPMI910B7w&pli=1

Peter Lemkin
08-20-2012, 03:14 PM
The range and scope of British and American SIGINT operations in Cyprus
during the first decades of the Cold War become more evident after a very
recent revelation concerning the mysterious circumstances surrounding the
death of the UN Secretary General Dag Hammarskjold in a supposed plane
accident on 18 September 1961 in Northern Rhodesia - now Zambia. An
American Intelligence officer stationed at the NSA Naval Security Group
Command installation at Yerolakkos, 7 west of Nicosia, recounts how he
intercepted the transmission of the pilot of another plane in the area of the
subsequent crash, closing in on the DC6 with Hammarskjold on board, firing
his plane’s guns and then shouting "I've hit it". 8 The American officer was thus
able to monitor clearly the apparent attack that took place 5,340 km away.

8 Susan Williams, Who Killed Hammarskjold?, C Hurst & Co (Publishers) Ltd, 2011, pp. 139-150.

Very important information! I'm sure Hammarskjold was assassinated. Just think how it must feel to those men behind the curtains to have murdered a US President, A UN Secretary General, many, many others and started all kinds of wars and black ops, overthrown nations....with impunity. The Swedes never really investigated it, nor did anyone else. A short UN-sponsored investigation claimed it an unfortunate 'accident'. Oh, and funny things related to Lockerbee also happened in Cyprus.

Dag Hammarskjöld: evidence suggests UN chief's plane was shot down

Eyewitnesses claim a second aircraft fired at the plane raising questions of British cover-up over the 1961 crash and its causes

Julian Borger and Georgina Smith in Ndola
guardian.co.uk, Wednesday 17 August 2011 19.20 BST

New evidence has emerged in one of the most enduring mysteries of United Nations and African history, suggesting that the plane carrying the UN secretary general Dag Hammarskjöld was shot down over Northern Rhodesia (now Zambia) 50 years ago, and the murder was covered up by British colonial authorities.

A British-run commission of inquiry blamed the crash in 1961 on pilot error and a later UN investigation largely rubber-stamped its findings. They ignored or downplayed witness testimony of villagers near the crash site which suggested foul play. The Guardian has talked to surviving witnesses who were never questioned by the official investigations and were too scared to come forward.

The residents on the western outskirts of the town of Ndola described Hammarskjöld's DC6 being shot down by a second, smaller aircraft. They say the crash site was sealed off by Northern Rhodesian security forces the next morning, hours before the wreckage was officially declared found, and they were ordered to leave the area.

The key witnesses were located and interviewed over the past three years by Göran Björkdahl, a Swedish aid worker based in Africa, who made the investigation of the Hammarskjöld mystery a personal quest since discovering his father had a fragment of the crashed DC6.

"My father was in that part of Zambia in the 70s and asking local people about what happened, and a man there, seeing that he was interested, gave him a piece of the plane. That was what got me started," Björkdahl said. When he went to work in Africa himself, he went to the site and began to question the local people systematically on what they had seen.

The investigation led Björkdahl to previously unpublished telegrams – seen by the Guardian – from the days leading up to Hammarskjöld's death on 17 September 1961, which illustrate US and British anger at an abortive UN military operation that the secretary general ordered on behalf of the Congolese government against a rebellion backed by western mining companies and mercenaries in the mineral-rich Katanga region.

Hammarskjöld was flying to Ndola for peace talks with the Katanga leadership at a meeting that the British helped arrange. The fiercely independent Swedish diplomat had, by then, enraged almost all the major powers on the security council with his support for decolonisation, but support from developing countries meant his re-election as secretary general would have been virtually guaranteed at the general assembly vote due the following year.

Björkdahl works for the Swedish international development agency, Sida, but his investigation was carried out in his own time and his report does not represent the official views of his government. However, his report echoes the scepticism about the official verdict voiced by Swedish members of the commissions of inquiry.

Björkdahl concludes that:

• Hammarskjöld's plane was almost certainly shot down by an unidentified second plane.

• The actions of the British and Northern Rhodesian officials at the scene delayed the search for the missing plane.

• The wreckage was found and sealed off by Northern Rhodesian troops and police long before its discovery was officially announced.

• The one survivor of the crash could have been saved but was allowed to die in a poorly equipped local hospital.

• At the time of his death Hammarskjöld suspected British diplomats secretly supported the Katanga rebellion and had obstructed a bid to arrange a truce.

• Days before his death, Hammarskjöld authorised a UN offensive on Katanga – codenamed Operation Morthor – despite reservations of the UN legal adviser, to the fury of the US and Britain.

The most compelling new evidence comes from witnesses who had not previously been interviewed, mostly charcoal-makers from the forest around Ndola, now in their 70s and 80s.

Dickson Mbewe, now 84, was sitting outside his house in Chifubu compound west of Ndola with a group of friends on the night of the crash.

"We saw a plane fly over Chifubu but did not pay any attention to it the first time," he told the Guardian. "When we saw it a second and third time, we thought that this plane was denied landing permission at the airport. Suddenly, we saw another aircraft approach the bigger aircraft at greater speed and release fire which appeared as a bright light.

"The plane on the top turned and went in another direction. We sensed the change in sound of the bigger plane. It went down and disappeared."

At about 5am, Mbewe went to his charcoal kiln close to the crash site, where he found soldiers and policemen already dispersing people. According to the official report the wreckage was only discovered at 3pm that afternoon.

"There was a group of white soldiers carrying a body, two in front and two behind," he said. "I heard people saying there was a man who was found alive and should be taken to hospital. Nobody was allowed to stay there."

Mbewe did not forward with that information earlier because he was never asked to, he said. "The atmosphere was not peaceful, we were chased away. I was afraid to go to the police because they might put me in prison."

Another witness, Custon Chipoya, a 75-year-old charcoal maker, also claims to have seen a second plane in the sky that night. "I saw a plane turning, it had clear lights and I could hear the roaring sound of the engine," he said. "It wasn't very high. In my opinion, it was at the height that planes are when they are going to land.

"It came back a second time, which made us look and the third time, when it was turning towards the airport, I saw a smaller plane approaching behind the bigger one. The lighter aircraft, a smaller jet type of plane, was trailing behind and had a flash light. Then it released some fire on to the bigger plane below and went in the opposite direction.

"The bigger aircraft caught fire and started exploding, crashing towards us. We thought it was following us as it chopped off branches and tree trunks. We thought it was war, so we ran away."

Chipoya said he returned to the site the next morning at about 6am and found the area cordoned off by police and army officers. He didn't mention what he had seen because: "It was impossible to talk to a police officer then. We just understood that we had to go away," he said.

Safeli Mulenga, 83, also in Chifubu on the night of the crash, did not see a second plane but witnessed an explosion.

"I saw the plane circle twice," he said. "The third time fire came from somewhere above the plane, it glowed so bright. It couldn't have been the plane exploding because the fire was coming on to it," he said.

There was no announcement for people to come forward with information following the crash, and the federal government did not want people to talk about it, he said. "There were some who witnessed the crash and they were taken away and imprisoned."

John Ngongo, now 75, out in the bush with a friend to learn how to make charcoal on the night of the crash, did not see another plane but he definitely heard one, he said.

"Suddenly, we saw a plane with fire on one side coming towards us. It was on fire before it hit the trees. The plane was not alone. I heard another plane at high speed disappearing into the distance but I didn't see it," he said.

The only survivor among the 15 people on board the DC6 was Harold Julian, an American sergeant on Hammarskjöld's security detail. The official report said he died of his injuries, but Mark Lowenthal, a doctor who helped treat Julian in Ndola, told Björkdahl he could have been saved.

"I look upon the episode as having been one of my most egregious professional failures in what has become a long career," Lowenthal wrote in an email. "I must first ask why did the US authorities not at once set out to help/rescue one of their own? Why did I not think of this at the time? Why did I not try to contact US authorities to say, 'Send urgently an aircraft to evacuate a US citizen on secondment to UN who is dying of kidney failure?'"

Julian was left in Ndola for five days. Before he died, he told police he had seen sparks in the sky and an explosion before the crash.

Björkdahl also raises questions about why the DC6 was made to circle outside Ndola. The official report claims there was no tape recorder in the air traffic control tower, despite the fact that its equipment was new. The air traffic control report of the crash was not filed until 33 hours afterwards.

According to records of the events of the night, the British high commissioner to the Rhodesian and Nyasaland Federation, Cuthbert Alport, who was at the airport that evening, "suddenly said that he had heard that Hammarskjöld had changed his mind and intended to fly somewhere else. The airport manager therefore didn't send out any emergency alert and everyone simply went to bed."

The witness accounts of another plane are consistent with other insider accounts of Hammarskjold's death. Two of his top aides, Conor Cruise O'Brien and George Ivan Smith, both became convinced that the secretary general had been shot down by mercenaries working for European industrialists in Katanga. They also believed that the British helped cover up the shooting. In 1992, the two published a letter in the Guardian spelling out their theory. Suspicion of British intentions is a recurring theme of the correspondence Björkdahl has examined from the days before Hammarskjöld's death.

Formally, the UK backed the UN mission, but, privately, the secretary general and his aides believed British officials were obstructing peace moves, possibly as a result of mining interests and sympathies with the white colonists on the Katanga side.

On the morning of 13 September the separatist leader Moise Tshombe signalled that he was ready for a truce, but changed his mind after a one-hour meeting with the UK consul in Katanga, Denzil Dunnett.

There is no doubt that at the time of his death Hammarskjöld‚ who had already alienated the Soviets, French and Belgians, had also angered the Americans and the British with his decision to launch Operation Morthor against the rebel leaders and mercenaries in Katanga.

The US secretary of state, Dean Rusk, told one of the secretary general's aides that President Kennedy was "extremely upset" and was threatening to withdraw support from the UN. The UK , Rusk said, was "equally upset".

At the end of his investigation Björkdahl is still not sure who killed Hammarskjöld, but he is fairly certain why he was killed: "It's clear there were a lot of circumstances pointing to possible involvement by western powers. The motive was there – the threat to the west's interests in Congo's huge mineral deposits. And this was the time of black African liberation, and you had whites who were desperate to cling on.

"Dag Hammarskjöld was trying to stick to the UN charter and the rules of international law. I have the impression from his telegrams and his private letters that he was disgusted by the behaviour of the big powers."

Historians at the Foreign Office said they could not comment. British officials believe that, at this late date, no amount of research would conclusively prove or disprove what they see as conspiracy theories that have always surrounded Hammarskjöld's death.