Sedro-Woolley Police Chief Doug Wood warned people to be on the lookout for Trevor Braymiller, The Seattle Times reported.
"He's considered armed and dangerous," Wood said. "People need to be alert and keep their eyes open."
Investigators initially believed the boy, whose name had not been released, had accidentally shot himself in the head but further examination showed the gunshot was not self-inflicted, Wood said.
The 5-foot-8, 165-pound Braymiller has brown hair and facial hair. He was last seen wearing a red or orange T-shirt, the Times said. It wasn't reported what crime Braymiller had been convicted of committing.
Wood said there were "no actual witnesses to the shooting" early Sunday though the boy's mother and another child were in the residence at the time.