An Army Blackhawk helicopter crashed on a mountain southeast of Seattle during a training mission, killing all three people aboard, military 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.

Searchers found the first two bodies quickly, then discovered the third Friday morning in the same general area, said Lt. Col. Dan Williams. He said the remains will undergo autopsies.

The names of the dead were not immediately released.

The soldiers were members of the 4th Squadron, 6th Air Cavalry Regiment, which has been at Fort Lewis for about a year, said David Kuhns, a civilian spokesman with the post.

Investigators were trying to piece together what caused the helicopter to crash. "It will be a lengthy process, but we aren't going to put a time limit on safety," Williams said.

King County sheriff's deputies reached the crash site about an hour after the helicopter went down on the south side of 1,835-foot Mount Peak near the county fairgrounds, about 30 miles east of Fort Lewis, said Deputy Rodney C. Chinnick.

The Army began using UH-60 Blackhawks in 1979 to provide air assault, medical evacuations and special operations support for combat operations. In August, a Blackhawk helicopter crashed in Iraq's Anbar province, killing two and injuring four.