On a winter day in 1985, a young Caucasian mother named Diana Robertson, age 21, ventured into the mountains near Tacoma, Washington, an outing that would become her last. On December 12th, she, her 36-year-old boyfriend Mike Riemer, and their two-year-old daughter, Crystal, set off in Riemer’s pickup truck to find a Christmas tree and for him to check his animal traps.
The day took a dark turn when their daughter was found wandering alone that afternoon outside a Kmart in Spanaway, miles from the forested area they had gone to visit. When asked about her parents, the toddler could only offer the chilling words, "Mommy is in the trees."
The disappearance of the couple launched a massive search effort, but for two months, there was no trace of Diana or Mike. The mystery of their whereabouts ended tragically on February 18, 1986, when a person walking their dog deep in a forest near Elbe discovered Diana’s body. She had been brutally stabbed 17 times, and a tube sock was tied tightly around her neck. Nearby was Riemer's abandoned truck. Inside, investigators found bloodstains on the front seat and an envelope with the message, "I love you, Diana," a detail that only deepened the mystery. There was, however, no sign of Mike Riemer.
Suspicion immediately fell upon Mike Riemer. Investigators learned that the couple had a volatile relationship, and Riemer had a documented history of domestic violence against Diana. A restraining order was even in effect at the time of their disappearance. This led to the primary theory that Riemer had killed Diana, abandoned their daughter at the store, and fled. A warrant was eventually issued for his arrest. However, another sinister possibility emerged when police linked Diana’s murder to the unsolved killings of another couple, Stephen Harkins and Ruth Cooper, four months earlier just 15 miles away. Ruth Cooper had also been found with a tube sock tied around her neck with a knot identical to the one used on Diana, suggesting a serial killer could be responsible.
For decades, the case remained cold, with two prevailing theories: either Riemer was a killer on the run, or he was the final victim of an unknown predator. The narrative shifted dramatically in 2011 when hikers discovered a partial human skull about a mile from where Diana’s body had been found. Dental records confirmed the skull belonged to Mike Riemer. This discovery led investigators to consider him a possible victim as well, though his cause of death could not be determined from the remains. To this day, no arrests have been made in Diana Robertson's murder, and the question of whether her killer was her abusive boyfriend or an elusive serial killer remains unanswered.
