Six people were arrested following the bloodshed at Lake Sammamish State Park, about 20 miles east of downtown Seattle, King County Sheriff's deputy John Urquhat said.
The violence erupted as a group of people gathered for "a party," Urquhat told The Associated Press.
A park ranger made an emergency call about around 9 p.m., and during the call more shots were fired.
As police flooded the scene, they found parkgoers 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 weren't immediately available.
"Our victims, we don't know if they're innocent victims, people caught in a crossfire, suspects, or other shooters," Urquhart told The Seattle Times. "It's quite possible there was more than one shooter."
Police were still interviewing witnesses late into the night. The park was closed and would likely remain that way through Sunday morning as authorities investigated and searched for weapons that might have been hidden in brushy areas, police 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.
Dawn Hilliker told The Times that she and her two teenage sons spent the day there celebrating a friend's high school graduation. They were packing up their car to leave when they heard several loud bangs from 250 yards away.
"I thought some kids were setting off firecrackers, but 30 seconds later, I saw three cops driving down the street," her son, Austin Hilliker, said. "I saw three people arrested ... It was super scary. It's shocking."
The family waited two hours to leave the park, as police stopped every vehicle on the way out, Dawn Hilliker said.
Hilliker said she taught her sons to stay away from places where they might encounter gang violence.
"You don't think coming to a graduation celebration at a state park that you would enter into that kind of environment. We came here to celebrate and ended up a part of chaos."