An Army Blackhawk helicopter crashed on a mountain southeast of Seattle during a training mission, killing two people and leaving a third missing, officials said Friday.

The UH-60 went down about 9 p.m. Thursday, according to a statement issued by Fort Lewis, an Army post south of Tacoma.

King County sheriff's deputies reached the crash site about 10 p.m. on the south side of 1,835-foot Mount Peak near the county fairgrounds, about 30 miles east of the military base, Deputy Rodney C. Chinnick said.

Army officials were investigating the crash, and about 20 deputies and dozens of volunteers were helping to search for the missing crewmember, Chinnick said.

It wasn't immediately clear what caused the helicopter to crash. Three people had been aboard, the Army said.