The 28-year-old Mount Vernon man was arrested about three hours after the discovery of the body in the parking lot of a Haggens Grocery Store on Thursday afternoon.
Police have not yet released the names of the victim or the man who was arrested. They are calling it a "suspicious death."
The investigation originally began Monday at about 10:30 p.m. when police received a call about shots fired in the 300 block of Anderson Road in Mount Vernon.
Officers responded to that call but didn't find anything suspicious. The next day, a man who lives in that area called police to say he had found blood in his house, which he shares with several people.
Mount Vernon police began looking for a car associated with that house and also asked Arlington police to be on the lookout for the vehicle. Police say the car they were looking for is the same car found Thursday with a body inside it.
Further investigation led them to the 28-year-old Mount Vernon man, who was believed to be with the victim Monday night at the time shots were fired. He was booked into Skagit County Jail for investigation of second-degree manslaughter.
Evidence collected by investigators indicates a shooting occurred at the Anderson Road residence, however the circumstances surrounding what occurred is still under investigation.
Anyone with information is asked to call the Mount Vernon Police Department's Criminal Investigations Division at (360) 336-6271.