TACOMA, Wash. (AP) — Four service members were aboard a U.S. Army MH-60 Black Hawk helicopter that crashed near Joint Base Lewis-McChord in Washington state, the Army said in a release Thursday. The Army did not release their conditions.

The helicopter was on a routine training mission when it crashed at about 9 p.m. Wednesday, an Army official said in a statement. The soldiers were part of the 160th Special Operations Aviation Regiment, Airborne, officials said. The cause of the crash was under investigation, the Army said.

“This remains an active, ongoing situation,” the release said.

The crash sparked a small wildfire that had grown to 1 acre (0.4 hectares) by Thursday morning, the Washington Department of Natural Resources said.

This is “currently a search mission” involving the “most professional and skilled addressing the situation,” Army spokesperson Ruth Castro told The Associated Press in an email. They were fully cooperating with law enforcement, she said. No details were released about the helicopter.

The base is about 10 miles (16 kilometers) south of Tacoma under the jurisdiction of the U.S. Army Joint Base Headquarters.

The Thurston County sheriff’s office, based in Olympia, posted online late Wednesday that deputies were dispatched to reports of a possible helicopter crash in the Summit Lake area, west of Olympia.

“We have been advised that the military lost contact with a helicopter in the area,” the department said. It said it was working with the base and that no further details were available.

Thurston County Sheriff Derek Sanders said on Facebook that deputies located the crash site, “but have been unable to continue rescue efforts as the scene is on fire.”

The King County Guardian 1 helicopter and special operation rescue units responded to the crash site, the sheriff said.

Staff with the state natural resources were working with multiple agencies, including the military and the local fire protection district, to battle the fire started by the crash, said Thomas Kyle-Milward, DNR wildfire communications manager.