Investigators believe the suspect, 36-year-old Benjamin Charles Parker, killed himself, the Washington County Sheriff's Office said in a news release on Sunday evening. Officials said there was no ongoing threat to the public.
They did not say how investigators were led to Parker, or what connection he had to the unnamed victims.
The sheriff's office said earlier that authorities responded early on Saturday to reports of a vehicle fire at a property in Forest Grove, about 50km west of downtown Portland. Responders then found the remains of five people and "several domestic animals".
They did not disclose the ages or genders of the people found, how they died or what possible connections they might have with one another.
At a news conference on Sunday afternoon, sheriff's Detective Shannon Wilde said they were still working to determine who the victims were.
"Any time you have one person deceased, let alone five, that's a logistical challenge," Wilde said, noting the complexity of the investigation and the large crime scene.
When asked by reporters about the amount of time that passed between authorities first discovering the bodies and releasing information the following afternoon, Wilde said authorities were trying to identify the victims and notify their next of kin while also working on a "very active investigation with a lot of moving pieces".
Authorities wanted to ensure they released accurate information in a responsible way, she said.
Sheriff Caprice Massey urged anyone who is familiar with the property or the people who live there to share any information they may have.
"I will say that in my 22 years here, I have not been a part of or witnessed a scene quite this complex involving this type of an investigation," she said.
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