Three children and two adults were killed, the King County Sheriff's department said in a statement. A girl who lived in the house was also injured, Reuters reported.
Sheriff's department spokesperson Mike Mellis told reporters at the scene that the suspect was taken into custody by deputies without incident.
It was not immediately clear if all those involved were members of the same family, and the sheriff's department said it was still working to determine a motive for the violence.
The sheriff's department said that deputies responded to multiple calls about gunfire at a home in Fall City, about 10 miles east of Seattle, early on Monday. A neighbor "with some medical training was able to administer first aid before first responders arrived," the department said.
The boy taken into custody was booked at the King County juvenile detention facility and is expected to make a first court appearance on Tuesday or Wednesday, the sheriff's department said.