At least one gunman opened fire during a party at a state park near Seattle, killing two men and wounding four others, police have said.
Six people were arrested following the bloodshed at Lake Sammamish State Park, about 20 miles east of central Seattle, King County Sheriff's deputy John Urquhat said.
The violence erupted as a group of people gathered for "a party" on Saturday night, Mr Urquhat said.
A park ranger made an emergency call about around 9pm, and during the call more shots were fired.
As police flooded the scene, they found members of the public hiding in park buildings to get away from the gunfire. "At this point, we don't know what precipitated the shooting," he said.
The wounded were taken to a Seattle hospital. Their conditions and the names of the dead and injured were not immediately available.
"Our victims, we don't know if they're innocent victims, people caught in a crossfire, suspects, or other shooters," Mr Urquhart said. "It's quite possible there was more than one shooter."
Police were still interviewing witnesses early on Sunday. The park was closed and would probably remain that way as officers investigated and searched for weapons that might have been hidden in brushy areas, they said.
Police said one gun was recovered from the scene and several more guns were found in cars attempting to leave the park, a popular gathering place for boating and watersport activities.