Six people were arrested following the gunfire at Lake Sammamish State Park, about 20 miles east of central Seattle, King County Sheriff's deputy John Urquhat said yesterday.
The violence erupted as a group of people gathered for a party on Saturday night, he 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 yesterday. Officers were searching the park for weapons that might have been hidden in the bushes.
Police said one gun was recovered from the scene and several more guns were found in cars trying to leave the park, a popular gathering place for boating and watersport activities.
Dawn Hilliker said she and her two teenage sons were there celebrating a friend's high school graduation. They were packing up their car to leave when they heard several loud bangs.