(The Hill) — The Air Force on Wednesday grounded the entire fleet of CV-22 Ospreys after a fatal crash in Japan on Nov. 29 killed eight U.S. servicemembers.

Lt. Gen. Tony Bauernfeind, command of Air Force Special Operations Command (AFSOC), directed an operational grounding of the CV-22 fleet to “mitigate risk” while the investigation into the crash continues, AFSOC said in a statement.

A preliminary review of the crash indicates a “potential materiel failure” caused the accident, according to AFSOC, but the cause is still unknown and the investigation is ongoing.

“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 returns to flight operations,” AFSOC said.

Japan had asked the U.S. to suspend its Osprey flights following the crash, but the Pentagon had said only the Air Force unit involved with the crash had suspended its flights.

Japanese officials have repeatedly expressed alarm about the hybrid aircraft. In 2016, a U.S. Osprey crashed near Okinawa, prompting an initial round of safety concerns.

The CV-22 Osprey crashed near Yakushima island in southwestern Japan, with eight airmen aboard. One of the aircraft’s engines reportedly burst with fire before it plummeted into the water.

The Osprey had taken off from Yokota Air Base in Tokyo, where U.S. forces operate as part of an agreement with Japan.

About a week out from the crash, rescue teams have transitioned to a search and recovery mission. Six of the remains of the fallen soldiers have now been recovered; two of the bodies have yet to be located.

The U.S. military has been plagued by Osprey crashes for years, and there are concerns the clutch may be behind some incidents.

A fatal crash that killed three in August led to the Marines ordering a safety review of the aircraft.