Two adults and three children were found dead after deputies responded early Monday to 911 calls of a "disturbance with gunfire," according to the King County Sheriff's Office.
The victims were Mark and Sarah Humiston and their three children, and the suspect is their 15-year-old son, CNN affiliate KING5 reported.
Mark was an electrical engineer and Sarah used to be a registered nurse, KING5 reported. The children killed were "young teenagers," Mike Mellis, a deputy and spokesperson for the sheriff's office, said in a Monday news conference.
Another child - a girl - was injured and taken to Harborview Medical Center in Seattle. She was in "satisfactory" condition on Tuesday, a hospital spokesperson told CNN.
"It does appear to be that this is a family incident, clearly a domestic violence incident involves not only a young man who's now in significant trouble, it involves firearms. Young men and firearms," Mellis told reporters Monday.
There have been at least 427 mass shootings this year in the United States, according to the Gun Violence Archive, which, like CNN, defines a mass shooting as one that injures or kills four or more people, not including the shooter.
The deaths in Washington state were "perpetrated by firearm" and "there was no significant confrontation" with the suspect when he was arrested, Mellis said.
The suspect waived his presence at his first hearing in juvenile court on Tuesday, according to his public defense attorneys.
Prosecutors have not formally charged him as the case has not yet been referred to them by law enforcement, according to Casey McNerthney, a spokesperson for the King County Prosecuting Attorney's Office.
In a statement, the teen's attorneys, Amy Parker and Molly Campera, described their client as a "15-year-old boy who enjoys mountain biking and fishing and has no criminal history."
"Again, the law says our client is presumed innocent of these charges, and we are grateful for a judicial process that requires proof and evidence before there is a judgment," the attorneys said.
A neighbor said she was "in total shock" after learning about the deaths.
"I keep bursting into tears," the neighbor, Lynne Trowern, told KING5. "That is why I have to go to my daughter's house, because I can't be here on my own. I just keep seeing the faces of the children."
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