WASHINGTON >> Secret until now, stealth helicopters may have been key to the success of the Osama bin Laden raid. But the so-far-unexplained crash of one of the modified Black Hawks at the scene apparently compromised at least some of the aircraft’s secrets.

The two choppers evidently used radar-evading technologies, plus noise and heat suppression devices, to slip across the Afghan-Pakistan border, avoid detection by Pakistani air defenses and deliver two dozen Navy SEALs into the al-Qaida leader’s lair. Photos of the lost chopper’s wrecked tail are circulating online — proving it exists and also exposing sensitive details.

President Barack Obama traveled Friday to Fort Campbell, Ky., and met privately with the elite Army pilots who flew the daring mission. They are members of the 160th Special Operations Aviation Regiment, nicknamed the Night Stalkers, and he saluted them in public remarks afterward.

The reason one of the helicopters crash landed at the bin Laden compound has not been disclosed, but Daniel Goure, a defense specialist at the Lexington Institute think tank, said Friday it might be explained by the unusual aerodynamics resulting from the aircraft’s modifications.

"It could be much more difficult to fly, particularly at slow speed and landing than you would expect from a typical Black Hawk," Goure said.

The U.S. military’s first stealth aircraft, the now-defunct F-117 fighter jet, was notoriously difficult to handle in flight, officials have said.

Night Stalker pilots also fly other, publicly acknowledged versions of the Black Hawk that are specially equipped with advanced navigation systems, plus devices allowing for low-level and all-weather flight, day or night. Those are rigged to permit occupants to "fast rope" from the helicopter as it hovers just off the ground — a technique used in the bin Laden assault.

Also taking part in the bin Laden mission were two MH-47 Chinooks, specially modified versions of the heavy-lift Chinook helicoptersthat are widely used by the Army’s conventional forces.

The MH-47s are flown by the 160th, the Night Stalkers. Those aircraft are not known to have stealth capabilities, although one was summoned to the scene of the raid after one of the stealthy Black Hawks crash-landed, in order to help ferry the SEAL contingent out of Pakistan.

Many aspects of stealth technology have been known for decades, including the use of angled aircraft edges and composite materials to make aircraft less visible on radar. The Army began a program to build a new class of helicopter with stealth technology in 1992. Known as the RAH-66 Comanche, it was canceled in 2004, in part to speed up development of drone aircraft.

Bill Sweetman, editor-in-chief of Defense Technology International and a long-time student of stealth aircraft development, said the biggest secret behind the stealth helicopter is simply that it existed.

"There was obviously a fairly high risk that you were going to compromise it one way or another the minute you used it," he said in an Associated Press interview.

The decision to use the helicopters reflected the extraordinary stakes involved in eliminating bin Laden, the world’s most-wanted terrorist. It is not known whether the choppers have been used in earlier Special Operations raids, but Dick Hoffman, a former Navy SEAL and now a defense analyst at the Rand Corp. think tank, said he had never before heard of their existence.

Hoffman said in a telephone interview that the apparent stealth technology on the choppers boosted the raid’s chances for success.

"Getting into the target area undetected is hugely important, especially with these terrorist targets and militia targets," he said. He noted that the SEAL team did not arrive at the Abbottabad compound in complete silence, since a resident in the same town was writing on Twitter during the raid that he could hear one or more helicopters and wondered what was happening.

But the modifications that suppressed noise from the helicopters — including the use of extra blades in the tail rotor and placement of a hubcap-like cover on the rotor — may have been sufficient to allow the assault teams to get on the ground before bin Laden and his security guards could mount enough of a defense to slow the SEALS; only one of the defenders was said to have gotten off a shot.

Noise suppression, Goure said, is "a huge advantage in these kinds of strikes."

Some elements of that noise suppression technology were visible in photos of the tail section that was left behind. The main body of the helo was blown up by the SEALs before they left with bin Laden’s body, apparently in order to prevent the exposure of other secret stealth components.

A Pentagon spokesman, Marine Col. David Lapan, declined to say Friday whether Pakistan was resisting U.S. efforts to retrieve the remains of the chopper.

Sweetman said it was remarkable that the SEALs managed to swoop into the compound and catch the bin Laden party by surprise.

"They’re probably expecting that someday they could get a visit from (U.S.) Special Forces," he said. "But they would also be expecting to hear helicopters for a few minutes before they arrive overhead. If your first warning is that you hear the thing and then you look up and it’s right there, you’ve lost valuable time."