The US is grounding its entire fleet of V-22 Osprey aircraft after eight service members were killed in a crash off the coast of Japan last week.
The Air Force Special Operations Command (Afsoc) said in a statement that the fatal crash was probably caused by material failure and not crew error.
Afsoc commander Lt Gen Tony Bauernfeind directed the shutdown “to mitigate risk” as the investigation into the crash continues.
Afsoc has 51 Ospreys and the Navy uses 27. The Marine Corps operates as many as 400.
The crash has renewed questions about the safety of the Osprey, which has been involved in several fatal accidents since being introduced into the US military in 2007. The aircraft has been used for missions in Iraq, Libya, Kuwait and elsewhere.
"This action has been taken out of abundance of caution while the Afsoc sock investigation is conducted," said Sabrina Singh, deputy Pentagon press secretary.
In August, three US service members were killed during a training exercise off the Australian coast. Last year, another fatal crash in Australia was caused by mechanical failure.
That same year, five US Marines were killed when an Osprey crashed in southern California, also because of mechanical failure.
In 2017, two US military personnel suffered non-life threatening injuries when an MV-22 Osprey made a hard landing in Syria, and a Marine Corps Osprey that was supporting a counter-terrorism operation crash-landed in Yemen.
The cause of a 2010 crash in Afghanistan that killed four people and injured 16 could not be determined because it did not meet the standard of “clear and convincing evidence”, Afsoc said at the time.
The MV-22B Osprey is a hybrid aircraft that is able to take off and land like a helicopter but fly like a conventional aeroplane.
The Bell Boeing V-22 Osprey has more 600,000 flight hours since being introduced. Bell Boeing describes it as "one of the most versatile and reliable aircraft" for US national security.