King County sheriff's spokeswoman Cindy West said the initial 911 call Sunday evening came from someone who reported that a man had been shot in the head inside a camper that was parked behind a home.
When deputies arrived at the house in the 26800 block of 172nd Place SE they found the 46-year-old victim dead at the scene and detained several people for questioning.
After interviewing witnesses, detectives arrested a 26-year-old man for investigation of murder and booked him into the King County Jail.
It was not clear what prompted the killing, but investigators said the suspect and victim knew each other.
The name of the man who was killed has not been released.