A Washington State Patrol explosives-sniffing dog found the gun Sunday under the stairs of a church about a half-block from the house where the child was shot that morning, the Skagit Valley Herald reported.
No shell casing was found at the scene, but there is one in the gun, indicating the gun misfired and didn't eject the shell. Lt. Lin Tucker said.
People at the house indicated the boy shot himself, but investigators suspect a homicide. The Skagit County coroner planned to examine the boy's body Monday.
Officers are looking for Trevor Braymiller, 25, the mother's boyfriend who fled the house. Police were told Braymiller went to the nearby church. A friend gave Braymiller a ride to the Big Lake community, about five miles south of Sedro-Woolley, in the Cascade foothills about 70 miles north of Seattle.
The friend went to police after learning the child was dead.
Braymiller is a felon, convicted of selling drugs, and not supposed to have a gun, said Sedro-Woolley police Chief Doug Wood. The house is well known to police, and officers conducted a drug raid there in 2011. Police have seized firearms from Braymiller in the past.
No one in the house saw the shooting, Tucker said. Also in the house at the time was the boy's mother, a couple, another young man and a girl about 2 years old who was parented by the mother and Braymiller.
"We're getting a lot of input from citizens saying they would like to find this guy and beat him up," Tucker told the newspaper. "We would rather they not turn our suspect into a victim."