Two adults and three young teenagers were killed after a "disturbance with gunfire" at a home in Fall City, east of Seattle, Mike Mellis, a deputy at the King County Sheriff's Office, said at a news conference this week.
A judge ruled on Tuesday that the 15-year-old suspect - whom media are not allowed to name - cannot have contact with a surviving victim in the incident, a teenager.
The sheriff's office has yet to name the victims in the shooting.
But several US media outlets have reported that parents Mark and Sarah Humiston and their three children were killed.
The injured victim, an 11-year-old girl, was in “satisfactory condition” at Harborview Medical Center in Seattle on Tuesday, a hospital spokesperson told the Associated Press.
Mr Mellis said police officers from all over the area responded to 911 calls about the gunfire and arrived to find a "chaotic scene" at the home, where a neighbour was providing medical aid to one of the teenage victims.
Police took a "young man" into custody shortly afterwards, Mr Mellis added.
“There was no significant confrontation with the young man who was taken into custody,” Mr Mellis said. “What had happened, had happened. It was done.”
The 15-year-old has no criminal history, his lawyers said in court on Tuesday. Officials have until Thursday to file charges against the teenager.
Gun violence is a fixture in American life - but the issue is a highly political one, pitting gun control advocates against people who are fiercely protective of their right to bear arms.
We've looked into some of the numbers behind firearms in the US.
There have been more than 385 mass shootings across the US so far this year, according to the Gun Violence Archive, which defines a mass shooting as an incident in which four or more people are injured or killed. Their figures include shootings that happen in homes and in public places.
For each of the last four years there have been more than 600 mass shootings - almost two a day on average.
He counts US shootings. He no longer remembers names
The deadliest such attack, in Las Vegas in 2017, killed more than 50 people and left 500 wounded. The vast majority of mass shootings, however, leave fewer than 10 people dead.
