The incident unfolded Sunday when deputies were called to a residence in Arlington by the woman's son, Joel Harris, who was concerned about his mother's disappearance, according to court documents.
Harris told deputies that his mother had disappeared the day after his grandmother had been found dead at the bottom of the basement stairs at her home, in the 13200 block of 240th Street NE, where his mother's brother also lived, court documents say.
Harris also said he had found his mother's vehicle, with full grocery bags still inside, on the property where the brother lived. At that point he became suspicious that the brother, later identified as David W. Thorsen, may have been involved in her disappearance.
That suspicion was heightened when he found whiskey, beer and a cocktail mix in the grocery bags in her car, none of which his mother drinks. He also found ATM receipts on the seat, and later noticed Thorsen dragging or towing something behind a riding lawn mower on the property, court records show.
Harris said when he went to speak to Thorsen about it, he found him high or drunk on drugs or alcohol. Thorsen told Harris he didn't know anything about his mother's whereabouts.
When police arrived at the scene to speak with Thorsen about his sister's disappearance, they found him reclining in his Honda Accord, a can of beer in his lap, with the engine running, the windows rolled up and a hose routed from the exhaust pipe of the car into the passenger compartment.
Police managed to wake Thorsen up and get him out of the car. He told detectives that his mother was gone, his father was gone and he wanted to go, too.
After police questioned Thorsen, a sergeant conducted a search of the property and discovered a woman's body in a field near the house covered with a blanket. The body was about 25 to 30 feet away from the where the riding lawn mower was parked, according to court documents.
Major Crimes Unit detectives were then called to the scene, said Shari Ireton of the Snohomish County Sheriff's Office.
Harris told detectives that there had been contention among family members about his grandfather's will and how property was to be divided up among them. He said his mother was part of that dispute.
He also told detectives that years before he had witnessed a fight between his mother and Thorsen. During the fight, Thorsen allegedly choked his mother nearly unconscious in a bathroom, but she was able to escape, court records show.
Detectives then interviewed neighbors. One of them said he had heard a woman's voice screaming, "Don't! Don't!" at about 2 p.m. on Sunday. Nothing else was heard so the neighbor did not investigate.
Investigators also saw what appeared to be a blood on Thorsen's pants and scratches on his hands and both legs, according to the case file. His clothes were taken as evidence.
Detectives determined there was reason to believe that Thorsen was responsible for his sister's death and had attempted to conceal her body on his property, Ireton said.
He was booked into the Snohomish County jail Sunday night for investigation of second-degree murder. In a Monday afternoon court hearing he pleaded not guilty and was ordered held on $1 million bail.
The name of the deceased sister was not immediately released.