The two rival Palestinian factions have failed to heal their political disputes since Hamas fighters expelled Fatah from Gaza in a short war in 2007
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.