Three other juveniles are in stable condition at a hospital.
The status of the suspect is unknown.
"As Spokane County officials continue their investigation to get the answers to the questions so many of us have during these unthinkable tragedies, the Washington State Patrol and all state agencies will do whatever we can to help provide support and comfort in the days and weeks to come," Washington Gov. Jay Inslee said in a statement.
He called the shooting "heartbreaking" and said all residents of the state are thinking