Unsolved Mysteries Wiki

Mike Riemer and Diana Robertson

Real Name: Diana Lee Robertson
Nicknames: No known nicknames
Location: Tacoma, Washington
Date: December 15, 1985


Details: On the morning of December 12, 1985, twenty-one-year-old Diana Robertson went with her thirty-six-year-old boyfriend Mike Riemer and their daughter Crystal into the woods near Tacoma, Washington to look for a Christmas tree. Mike, a trapper, also planned to check his animal traps in the area. Later that afternoon, Crystal was discovered wandering around a Tacoma department store without her parents. The store employees tried to find her parents, but were unsuccessful. After being checked at a local hospital, she was sent to a temporary foster home. Three days later, she was identified after her photo was shown during a news broadcast and her maternal grandmother Louise recognized her. She was then sent to live with her.
Mike and Diana had vanished without a trace; all Crystal could tell police is that "Mommy is in the trees". That weekend, an extensive search began for the couple. Mike's friends searched both the ground and the air for them, focusing on the areas where he set his traps. However, no trace of them was found. Two months later, Diana's body was found deep in the forest around Elbe, Washington. She had been stabbed seventeen times and had a tube sock tied around her neck. There was no sign of Mike at the scene.
Mike's family and friends believed that he was also killed. They believe his body was dumped nearby. However, other evidence at the scene seemed to point to him being involved. His truck, which was abandoned next to Diana's body, had blood stains on the front seat and note on the dashboard that read, "I love you, Diana." Although her mother believed that it was his handwriting, it could not be confirmed by the FBI laboratory. Tests on the blood confirmed that it was human; however, the blood type could not be determined.
Police believe that Diana's murder and Mike's disappearance may be related to a double murder that occurred in the same woods four months earlier. A man named Stephen Harkins was found shot to death in his sleeping bag. His companion, Ruth Cooper, was also found shot to death, two months later. They were camping in the same area where Mike set his traps, just fifteen miles from where Diana's body was found. Both Ruth and Diana had been killed with the tube sock tied in the same knot.
Two theories emerged surrounding the cases. One was that an unknown serial killer had killed both couples and hidden Mike's body. The other was that Mike himself was the serial killer. He apparently had a history of domestic violence. According to friends and family, she was frequently beaten by him. In October 1985, he was arrested after he allegedly kicked down her apartment door, threw her to the floor, and rubbed her face in the carpet. A few weeks before her murder, Diana secured a court order, keeping Mike from seeing her. However, as Christmas approached, the couple reconciled.
Diana allegedly told her mother that Mike had threatened to kill her and that he would get away with it. However, his family and friends are convinced that he is innocent. Although they admit he had a temper, they do not believe that he would be capable of murder. Because of the cold temperatures, his father does not believe that Mike would leave his coat behind in the truck. He believes that Mike was killed first.
The circumstantial evidence was enough for police to issue a warrant for his arrest. However, police have been unable to prove if he is alive or dead.
Suspects: Mike was the prime suspect in Diana's, Stephen's, and Ruth's murders. At the time of his disappearance, he was 6'0", 180 pounds, has blue eyes, brown hair, a scar on his upper lip, and was a skilled outdoors man who was thought to be living off the land. However, others believed that an unidentified serial killer murdered the couple, as other couples were murdered before Diana and Mike.
Extra Notes: This case first aired on the September 6, 1989 episode. It was excluded from Amazon Prime episodes.
Results: Unresolved. In March 2011, Mike's skull was found just one mile from where Diana's body was found. The cause of his death is unknown. Authorities suspected that an unknown serial killer may be responsible for the Harkins/Cooper and Robertson/Riemer murders.
According to the "Washed Away" Podcast, a DNA profile was found on the ligature used to tie Diana's hands. Cold case investigators are currently trying to compare Mike's DNA to the other profile.