Three people were killed and another injured in a shooting at a house party in Mukilteo overnight, about 25 miles (40 km) north of Seattle, officials said.
A suspect was later taken into custody at a traffic stop early Saturday, police told reporters.
"Our community has been shaken to its core ... There are many unanswered questions about what happened tonight," Mukilteo Mayor Jennifer Gregerson said at the same news conference.
Many of the witnesses were young, she added.
The names and ages of those killed and details on the suspect were not released.