Al-Qaeda leader Bin Laden 'dead'. Again.

[Jan Klimkowski]

Whatever the "raid" was, it appears that a Blackhawk helicopter modified with Stealth technology was used, and - highly embarrassingly for the US military - crashed or was shot down.

Osama bin Laden death: photos suggest use of 'stealth' helicopters

Blackhawk wreckage reveals previously unseen modifications, seemingly to reduce radar visibility and muffle noise

It is yet another question over the operation that killed Osama bin Laden: did the US Navy Seal team sneak up to his compound using previously top-secret "stealth" helicopters?

A number of military analysts believe photographs of the wreckage of one of the two Blackhawk helicopters used in the raid, which the Seals blew up after it was damaged, show hitherto unknown modifications to the aircraft, seemingly to reduce its radar visibility and muffle noise.

One former US special operations air expert told the Army Times that the distinctive hard-angled shape of the destroyed Blackhawk resembled the fuselage of the F-117 Stealth Fighter, which is designed to deflect radar. "It really didn't look like a traditional Black Hawk," he told the US forces' newspaper.

The US did not warn Pakistan about the raid for fear of leaks, but the helicopters nonetheless managed to fly to the compound from their base in Afghanistan without Pakistan's military seemingly being alerted.

Other photos appeared to show a circular cover over the helicopter's tail rotor, which experts said could be intended to dampen noise, making it harder for people on the ground to anticipate the aircraft's arrival. Some of those living near the compound said they did not hear the helicopters until they were almost directly overhead.

The military team that killed the al-Qaida leader arrived in two helicopters described officially only as Blackhawks, the Sikorsky-made US military's workhorse helicopter for the past 30 years. One was damaged as it landed heavily at Bin Laden's compound, reportedly after its rotor clipped a wall, and the Seals destroyed it with explosives before departing in the other helicopter.

Stealth aircraft canopies are covered in a radar-deflecting coating and it was "very plausible" that this could have made the Blackhawks harder to fly at night, perhaps causing the crash, the ex-special operations expert told the Army Times.

The US military has previously experimented with stealth helicopters, notably an adapted version of the Comanche reconnaissance aircraft. However none have previously been seen on active service. The Pentagon has refused to comment on questions about the Blackhawks used in the Bin Laden raid.

Dan Goure, a former Pentagon official now with the Lexington Institute thinktank, told ABC News: "This is a first. You wouldn't know that it was coming right at you. And that's what's important, because these are coming in fast and low, and if they aren't sounding like they're coming right at you, you might not even react until it's too late ... That was clearly part of the success."