Investigators believe the man and the victim were involved in a shooting in Mount Vernon on Monday.
The man was being held for investigation of second-degree manslaughter, court papers show.
A Skagit County judge on Friday ordered the arrest report sealed to protect the ongoing investigation, court records show.
Mount Vernon police were called Monday night after somebody heard gunshots near a home along Anderson Road. Nothing suspicious was found.
Multiple people lived at the home. One of them returned Tuesday after being away and thought something seemed wrong, Mount Vernon police Lt. Chris Cammock said.
"Blood and other suspicious circumstances were found inside, and the resident became concerned that possibly one of the other occupants had been injured," he said.
Detectives developed information leading them to believe a car connected to the case might be in the Arlington area.
Arlington police found the car in the Haggen parking lot near Highway 9 about 3 p.m. Thursday. A man's body was in the passenger seat. Mount Vernon police are calling his death suspicious.
The man's identity was not released Friday. His cause of death remained under investigation, according to the Snohomish County Medical Examiner's Office.
An autopsy was conducted Friday, Cammock said. Detectives also were in Snohomish County collecting potential evidence from the car. Washington State Patrol crime-scene specialists were assisting.
Police ask anyone with information to call 360-336-6271.
Rikki King: 425-339-3449; firstname.lastname@example.org
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