The US Navy C-2 Greyhound cargo plane that crashed off Okinawa in the Philippine Sea on Wednesday was ferrying passengers to the carrier USS Ronald Reagan which was then engaged in a joint exercise with ships of Japan’s Maritime Self-Defense Force (JMSDF).

It was the fifth serious accident involving the US Navy this year in Asian waters. A total of 17 US sailors have died in the incidents which are raising questions about Navy preparedness and training as military activity escalates in the face of a nuke crisis with North Korea.

US Navy statements indicated that all on board the aircraft were sailors, according to USNI News. There had been speculation that the aging propeller-driven C-2s, which are part of the service’s public outreach programs, might have been carryings VIPs or journalists to the Reagan as part of the high-profile maneuvers.

Eight of the passengers were recovered about an hour after the crash at 3:23 p.m. local time and were taken for medical evaluation aboard Reagan, according to USNI News. They are reportedly in good condition. Three others are missing and are the subject of a search by US Navy and JMSDF forces in the area.

“At approximately 2:45 p.m. Japan Standard Time, Nov. 22, 2017, the C2-A aircraft with 11 crew and passengers onboard crashed into the ocean approximately 500 nautical miles southeast of Okinawa. The aircraft was conducting a routine transport flight carrying passengers and cargo from Marine Corps Air Station Iwakuni to USS Ronald Reagan (CVN-76),” the US Navy’s 7th Fleet said in a statement.

An official investigation of the crash is underway. The vintage C-2 Greyhounds are some of the oldest planes in the US Navy and are based on a 50-year-old aircraft design. Used as aerial workhorses during the Cold War, they typically accompany US carrier strike groups, transporting items like mail and passengers in cases when distances are beyond the range of helicopters.

The C-2 crash follows on the heels of another mishap for the Navy on Saturday when the guided-missile destroyer USS Benfold was struck by a Japanese tug during a towing exercise in Sagami Bay about 80 km south of Tokyo. In this incident, the Navy says the tug lost power and, “drifted into” the destroyer.

In the most serious of earlier accidents, the destroyer USS John S. McCain and an oil tanker collided near Singapore in August, leaving 10 U.S. sailors dead. Seven American sailors also died in June when the USS Fitzgerald and a container ship collided off Japan.

Navy probes have concluded that all of the accidents were “avoidable” and stemmed from the inability of crews and commanders to recognize and respond quickly to emergency situations.

Eight top Navy officers, including the 7th Fleet commander, have been removed from their posts following the investigations.