WASHINGTON (AP) — The military announced late Wednesday it was grounding all of its Osprey V-22 helicopters, one week after eight Air Force Special Operations Command service members died in a crash off the coast of Japan.

The Air Force, Navy and Marine Corps took the extraordinary step of grounding hundreds of aircraft after a preliminary investigation of last week’s crash indicated that a materiel failure — that something went wrong with the aircraft — and not a mistake by the crew led to the deaths.

The crash raised new questions about the safety of the Osprey, which has been involved in multiple fatal accidents over its relatively short time in service. Japan grounded its fleet of 14 Ospreys after the crash.

Lt. Gen. Tony Bauernfeind, head of Air Force Special Operations Command, directed the standdown “to mitigate risk while the investigation continues,” the command said in a statement. “Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time.”

In a separate notice, Naval Air Systems Command said it was grounding all Ospreys. The command is responsible for the Marine Corps and Navy variants of the aircraft.

The Air Force said it was unknown how long the aircraft would be grounded. It said the standdown was expected to remain in place until the investigation has determined the cause of the Japan crash and made recommendations to allow the fleet to return to operations.

The U.S.-made Osprey is a hybrid aircraft that takes off and lands like a helicopter but can rotate its propellers forward and cruise much faster, like an airplane, during flight. Air Force Special Operations Command has 51 Ospreys, the U.S. Marine Corps flies more than 400 and U.S. Navy operates 27.

The Osprey is still a relatively young plane in the military’s fleet — the first Ospreys only became operational in 2007 after decades of testing. But more than 50 troops have died either flight testing the Osprey or conducting training flights in the aircraft, including 20 deaths in four crashes over the past 20 months.

An Osprey accident in August in Australia killed three Marines. That accident also is still under investigation.