The U.S. Air Force announced Wednesday that it is grounding its entire fleet of Osprey aircraft after investigators learned that the Osprey crash last week off the coast of Japan that killed all eight U.S. airmen aboard may have been caused by an equipment malfunction.

Lt. Gen. Tony Bauernfeind, commander of Air Force Special Operations Command, said in a statement that he ordered the "operational standdown" of all CV-22 Ospreys after a "preliminary investigation" indicated the crash may have been caused by "a potential materiel failure."

However, the exact cause of that failure is still unknown, Bauernfeind said.

"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," Bauernfeind said.

The move comes after Tokyo formally asked the U.S. military to ground its Ospreys in Japan until thorough inspections could be carried out to confirm their safety.

The Osprey, assigned to Yokota Air Base in Tokyo, was on a training flight when it crashed Nov. 29 off the southern Japanese island of Yakushima. It had departed from Marine Corps Air Station Iwakuni in Yamaguchi Prefecture and was headed to Kadena Air Base on Okinawa, but requested an emergency landing on Yakushima just before crashing off the shore.

Eyewitnesses said the aircraft flipped over and burst into flames before plunging into the ocean.

So far, the remains of three of the eight crew members have been recovered. Divers from both the U.S. and Japanese militaries earlier this week located a significant portion of the fuselage of the submerged wreckage, with the bodies of the remaining five crew members still inside.

There have been several fatal U.S. Osprey crashes in recent years. Most recently an aircraft went down during a multinational training exercise on an Australian island in August, killing three U.S. Marines and leaving eight others hospitalized. All five U.S. Marines on board another Osprey died in June of 2022 when the aircraft crashed in the California desert.

The Osprey is a tiltrotor aircraft used to move troops and supplies. It can take off and land like a helicopter, but can also fly like a plane.

— Lucy Craft, Tucker Reals and Elizabeth Palmer contributed to this report. 

In:
• Helicopter Crash
• U.S. Air Force
• Japan

Faris Tanyos is a news editor for CBSNews.com, where he writes and edits stories and tracks breaking news. He previously worked as a digital news producer at several local news stations up and down the West Coast.