The incident occurred about 2 a.m. after the planes took off from Marine Corps Air Station Iwakuni in Japan, officials said. The aircraft — an F/A-18 Hornet and a KC-130 Hercules — were conducting “regularly scheduled training” at the time, and Japanese authorities immediately launched search-and-rescue aircraft.
“The circumstances of the mishap are currently under investigation,” the statement said.
It was not clear immediately clear if the aircraft collided, or if something involving the refueling system itself sparked the mishap. Aerial refueling with the KC-130 calls for another aircraft to pull slightly below and behind the tanker plane, with hoses eventually extended from the KC-130. It can carry 3,600-gallon stainless steel refueling tanks on its wings or inside the fuselage, or share fuel from its own fuel tanks.
The KC-130 also can be used to transport ground troops. In July 2017, a KC-130T crashed in Mississippi while flying from Cherry Point, N.C., to Naval Air Facility El Centro in California, killing all 16 Marines on board and prompting the military to ground many other C-130s afterward.
The Marine Corps announced Wednesday night that an investigation into the crash in Mississippi found that an “in-flight departure” of one of the plane's propellers from a wing and into the aircraft's fuselage caused the disaster.
“The investigation determined that the aircraft’s propeller did not receive proper depot-level maintenance during its last overhaul in September 2011, which missed corrosion that may have contributed to the propeller blade liberating in-flight,” the service said in a news release.
Correction: This article has been updated to reflect accurate details about refueling.
This story was first published Wednesday night in Washington and has been updated several times with additional information.