Military aviation experts are now speculating that the mission that killed Osama bin Laden could owe part of its success to what appears to be a secret stealth helicopter, one of which was damaged and left behind in Pakistan.

Though US forces attempted to destroy the helicopter that was downed inside the compound's courtyard, its tail remained relatively intact on the other side of the wall. As pictures of the wreckage have emerged, aviation experts say the helicopter appears to share characteristics of both a Black Hawk helicopter and a stealth fighter jet.

The US was not known to have a stealth helicopter in its fleet, but if the helicopter used Sunday did indeed have stealth characteristics, it might have helped US forces to approach Mr. bin Laden’s compound in Abbottabad, Pakistan, undetected.

“It really didn’t look like a traditional Black Hawk,” a retired special operations aviator told the Army Times. The helicopter had “those distinctive edges and angles” seen on F-117 stealth fighters.

Despite apparent similarities, stealth helicopters and stealth jets have different design goals. Stealth jets are made for radar invisibility, while stealth helicopters are designed to be quiet. Helicopters already have the ability to fly below radar, and making them invisible to radar is difficult because they have so many moving parts.

“Reducing noise and making it less conspicuous is the first job,” writes Bill Sweetman in Aviation Week. This can be done by adding additional blades to “reduce the classic whop-whop signature” and by tweaking aerodynamics and flight controls to allow the pilot to lower the rotor speed at will.

To the untrained ear, a stealth helicopter might sound more like a lawnmower than a $150 million weapon.

The military’s last known stealth-helicopter program, the RAH-66 Comanche, was canceled in 2004 before it saw action in the field. The program spent $6.9 billion to produce two prototypes, and when it was canceled, military officials said innovations developed for Comanche would be used in future stealth crafts.

IHS Jane's, a leading source on the world's military aircraft, reported: "Images of the wreckage of a helicopter that reportedly crashed during the operation, apparently due to an undisclosed technical malfunction, do not conform to any types that are known to be in service with the US military or in development."

The Pentagon remains tight lipped about the crashed helicopter, and CIA Director Leon Panetta said Tuesday Black Hawk helicopters had been used in the operation. But experts say if this is indeed a new kind, it may have been in use for years.

The F-117 Nighthawk first flew in 1981 but was not revealed publicly until 1988. During the Vietnam War, the CIA used a light stealth helicopter called The Quiet One to gather intelligence. It was only used once and wasn’t known about until 1995.

Pakistani officials carted away the wrecked helicopter Monday, loading it into trucks and covering the pieces with tarps. Former White House counterterrorism adviser Richard Clarke said the US could have reason to worry about where that wreckage will end up, given Pakistan’s close relationship with China.

“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," Mr. Clarke said Wednesday on ABC’s Good Morning America.

A stealth-hungry military might be particularly interested a fabric-like material that covered the mysterious chopper, which is said to be key for shaping it for aerodynamics. Children were seen outside of bin Laden’s compound collecting pieces of the material this week.