The U.S. military has grounded all of its Osprey V-22 tilt-rotor aircraft a week after eight service members died when an Air Force CV-22 crashed off Kagoshima Prefecture’s Yakushima island in the first fatal accident for the type in Japan.

The U.S. Air Force, Navy and Marine Corps announced that they were grounding hundreds of aircraft after a preliminary investigation into the Nov. 29 crash indicated that problems with the aircraft — not a mistake by the crew — had led to the deaths.

“Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time,” Lt. Gen. Tony Bauernfeind, head of U.S. Air Force Special Operations Command, said in a statement late Wednesday.