A U.S. sailor was in critical condition Friday after being hit by a helicopter rotor blade at Camp Pendleton Marine Corps base, military officials said.

The unidentified sailor was struck by the spinning UH-1Y Venom helicopter tail rotor blade about 6:10 p.m. Wednesday.

Military officials declined to release further details, saying the incident was still under investigation.

The sailor was assigned to 34d Marine Aircraft Wing at Marine Corps Air Station Camp Pendleton, located north of San Diego.

The Associated Press contributed to this story.