WASHINGTON (AP) — A congressional oversight committee has launched an investigation into the V-22 Osprey program following a deadly crash in Japan which killed eight Air Force special operations service members.

The entire Osprey fleet remains grounded following the Nov. 29 crash with the exception of limited Marine Corps flights in emergencies. More than 50 U.S. service members have died in Osprey crashes over the lifespan of the program, and 20 of those died in four crashes over the last 20 months.

The Osprey is a fast-moving airframe that can fly like both a helicopter and an airplane — but its many crashes have led critics to warn it has fatal design flaws.

The government of Japan, the only international partner flying the Osprey, has also grounded its aircraft after the Nov. 29 crash.

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On Thursday the U.S. House of Representatives Committee on Oversight and Accountability sent a letter to Defense Secretary Lloyd Austin requesting a massive amount of documentation on the Osprey’s safety record to be delivered to the committee by Jan. 4.

“Our servicemembers remain in harm’s way without resolution of known mechanical issues, ” wrote the committee chairman, Kentucky Republican James Comer. “While, statistically, the Osprey is not considered as dangerous as some other military aircraft, the Committee remains alarmed that most fatalities involving the aircraft have happened during training exercises, not combat operations.”

The Osprey only became operational in 2007 after decades of testing. Since then, it’s become a workhorse for the Marine Corps and Air Force Special Operations Command, and was in the process of being adopted by the Navy to replace its C-2 Greyhound propeller planes, which transport personnel on and off aircraft carriers at sea.

Shortly after the Nov. 29 crash, the Air Force said that a malfunction of the aircraft, not a mistake by the crew, was probably the cause.

The Osprey has faced persistent questions about a mechanical problem with the clutch that has troubled the program for more than a decade. There also have been questions as to whether all parts of the Osprey have been manufactured according to safety specifications and, as those parts age, whether they remain strong enough to withstand the significant forces created by the Osprey’s unique structure and dynamics of tiltrotor flight.

Marine Corps Ospreys also have been used to transport White House staff, press and security personnel accompanying the president. White House National Security Council spokesman John Kirby said those Ospreys are also grounded.