Three of 26 US Marines on board a Bell Boeing MV-22 are still missing after the tiltrotor crashed off the east coast of Australia on 5 August, the III Marine Expeditionary Forces says in a news release.

The MV-22 assigned to VMM-265 squadron aboard the USS Bonhomme Richard “entered the water” during routine operations.

A search and rescue effort is continuing to find the missing three Marines.

The squadron is based at MCAS Futenma in Okinawa, but is embarked on the amphibious carrier as part of a joint exercise with Australian defence forces.

The potentially fatal mishap adds to a difficult year in Marine Corps aviation.

Through 4 August, the Marines’ manned aircraft fleet has logged a Class A mishap rate of 4.14 per 100,000 flight hours, according to data posted online by the Navy Safety Center.

Marine aircraft compiled a Class A mishap rate of 3.94 in fiscal 2016.

The mishaps this year include the loss of a KC-130T tanker, which killed all 16 aboard after departing controlled flight at cruise altitude over Mississippi and crashing in a field.

The accident could mark the first fatal MV-22 crash since May 2015, when two crew members died after the tiltrotor’s engine ingested too much dust and lost thrust in a hover over Bellows Field, Hawaii.