May 4, 2011  -- Before an elite team of U.S. Navy SEALs executed a daring raid that took down Osama bin Laden, the commandos were able to silently sneak up on their elusive target thanks to what aviation analysts said were top secret, never-before-seen stealth-modified helicopters.

In the course of the operation that cost the al Qaeda leader his life, one of the two Blackhawk helicopters that carried the SEALs into bin Laden's Pakistani compound grazed one of the compound's wall and was forced to make a hard landing. With the chopper inoperable, at the end of the mission the SEALs destroyed it with explosives.

But photos of what survived the explosion -- the tail section of the craft with curious modifications -- has sent military analysts buzzing about a stealth helicopter program that was only rumored to exist. From a modified tail boom to a noise reducing covering on the rear rotors and a special high-tech material similar to that used in stealth fighters, former Department of Defense official and vice president of the Lexington Institute Dan Goure said the bird is like nothing he's ever seen before.

"This is a first," he said. "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."

In addition to the noise-reducing modifications, a former special operations aviator told The Army Times the general shape of what was left of the craft -- the harsh angles and flat surfaces more common to stealth jets -- was further evidence it was a modified variant of the Blackhawk.

A senior Pentagon official told ABC News the Defense Department would "absolutely not" comment on anything relating to the destroyed bird.

Neighbors of bin Laden in Abbottabad, Pakistan, told ABC News they didn't hear the helicopters the night of the Sunday raid until they were directly overhead. The rotor covering, along with a special rotor design, suppressed the choppers noise while inbound, Bill Sweetman, editor and chief of Defense Technology International, said.

"Helicopters make a very distinctive percussive rotor sound which is caused by their rotor blades and if you can blend that down, of course that makes a noise that is much less likely to be heard and much more likely to blend into any background noise that there is," Sweetman said.

Parts of Helicopter Taken From Crash Site

The U.S. has attempted to use stealth helicopters before. In the mid-90s, the Army developed several prototypes of the Comanche helicopter, a reconnaissance helicopter that was at the time a revolutionary step in stealth technology. But in 2004 the Department of Defense scrapped the program and promised to use the technology developed for the Comanche on other crafts.

Since, the government has been working to silence the Army's Blackhawk helicopters but an official program for the stealth choppers was never publicized. The wreckage, Sweetman said, is the first the public has ever seen of an operational stealth-modified helicopter.

Goure said he believes the stealthy Blackhawks have been in use for years without the public's knowledge.

"We probably have been running hundreds of missions with these helicopters over the last half dozen years, and the fact is, they've all been successful -- or at least the helicopters have all come back," he said.

But now that one went down and photographs emerged of large sections being taken from the crash site under a tarp, former White House counterterrorism advisor and ABC News consultant Richard Clarke said U.S. officials may have reason to worry about where those parts end up.

"There are probably people in the Pentagon tonight who are very concerned that pieces of the helicopter may be, even now, on their way to China, because we know that China is trying to make stealth aircraft," he said. The Chinese military is known to have a close relationship with the Pakistani military.

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