The United States said on Wednesday it was grounding its military fleet of V-22 Osprey aircraft after a fatal crash last week off the coast of Japan that killed eight people onboard.

“Preliminary investigation information indicates a potential material failure caused the mishap, but the underlying cause of the failure is unknown at this time,” US Air Force Special Operations Command (AFSOC) said in a statement.

Eight American service members were aboard the tilt-rotor aircraft when it crashed during a routine training mission last week off Yakushima Island, about 1,040km (650 miles) south-west of the capital, Tokyo.

After the crash, the USmilitary unit that the V-22 Osprey aircraft belonged to suspended flight operations. But the US military had said other aircraft would continue to fly after undergoing safety checks.

Tokyo has voiced concern about Osprey flights. The deployment of the aircraft in Japan has been controversial, with critics of the US military presence in the country’s south-west islands saying it is prone to accidents.

“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.