The U.S. military is grounding its entire fleet of V-22 Osprey aircraft after investigators came to suspect a materiel failure, rather than human error, caused one to crash Nov. 29 off the coast of Japan, killing eight Air Force Special Operations Command service members.

The Navy and Marine Corps joined the Air Force in grounding their V-22 tilt-rotor aircraft while the investigation proceeds. Boeing and Bell, which jointly manufactured the aircraft, are “standing by to provide support” if requested by the military, the firms said in a statement to The Washington Post.

Lt. Gen. Tony Bauernfeind, head of the Command, issued the order to halt flights “to mitigate risk while the investigation continues,” he wrote Wednesday. “Preliminary investigation information indicates a potential materiel failure caused the mishap, but the underlying cause of the failure is unknown at this time.”