TOKYO — The U.S. military said Wednesday that it was grounding all of its Osprey V-22 helicopters, a week after one of the aircraft crashed off the coast of Japan, killing all eight U.S. airmen aboard.

A preliminary investigation of the crash of the CV-22B Osprey off the Japanese island of Yakushima on Nov. 29 indicates that it may have been caused by a materiel failure, the Air Force Special Operations Command said in a statement, meaning the problem was with the aircraft rather than crew member error. The underlying cause of the failure remains unknown.

The crash in Japan is the latest to raise safety questions about the Osprey, an American hybrid aircraft that has been involved in multiple fatal accidents.

Lt. Gen. Tony Bauernfeind, head of Air Force Special Operations Command, has ordered an operational standdown of the Air Force CV-22 fleet “to mitigate risk while the investigation continues.”

“The standdown will provide time and space for a thorough investigation to determine causal factors and recommendations to ensure the Air Force CV-22 fleet to return to flight operations,” the statement said.

The Naval Air Systems Command, which is responsible for both Marine Corps and Navy variants of the aircraft, said in a separate statement that it was grounding all V-22 Ospreys “out of an abundance of caution.”

The groundings affect hundreds of aircraft, according to the Associated Press, including 51 from Air Force Special Operations Command, 27 from the Navy and as many as 400 from the Marine Corps.

An Air Force official told NBC News it was unclear how long the standdown would last, but that the Ospreys would not return to the skies until they were deemed safe to fly.