The violence didn't appear to be random, but police still were trying to determine what happened, Cmdr. Robb Lamoureux said. Police don't believe there is any danger to the public.
The bodies were found by the grandfather, 82, who called 911 about 9 a.m. Friday, Lamoureux said.
The grandparents and the grandson lived in a mobile home at the Glenwood Mobile Estates park in the 5900 block of 64th Street NE.
For much of Friday, police blocked off roads in the park, a tree-lined retirement community where many people were working in their yards.
Investigators late Friday afternoon executed a search warrant on the property to collect evidence, Lamoureux said. They had not ruled out the possibility of a homicide-suicide, or a double homicide.
Detectives need to process the scene to determine how physical evidence fits with statements given to police, Lamoureux said.
The names of the people who lived at the home were not immediately released. The Snohomish County Medical Examiner's Office is expected to identify them and determine how they died.
Neighbors who spoke with reporters on Friday said they hadn't heard anything unusual before the police cars showed up. Many older people choose to live at the park because it feels safe and quiet, they said.
Police believe the grandson, 26, recently had moved in with his grandparents, Lamoureux said. The younger man reportedly suffered from mental health problems and had not been taking his medication.
Police would not say whether weapons were believed to be involved in the violence or disclose what they found at the home.
No arrests had been made as of early Friday evening.
"It's really early on in the investigation," Lamoureux said.
Rikki King: 425-339-3449; email@example.com.
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