Mark Porter told investigators that he had a close relationship with Gary and Jan Tyrrell, and he had been in their home a few days before they were found dead. He was requested to provide a sample of his DNA, but he refused. Investigators were suspicious of Porter, and they looked further into his relationship with the slain couple. It turned out that Porter had borrowed money from the couple and had yet to pay them, and he, again, tried to borrow an additional $250,000 but that time, Gary refused, according to the Springfield News-Leader.

Money may have been a motive for Porter to kill the couple, but that wasn’t enough to have him arrested. Further investigations revealed that Porter sold silver coins a couple of days after the couple was murdered, and he got $18,000 from the sale. Detectives kept Porter on their radar, and they were able to get a sample of his DNA from a discarded coffee cup. It was tested against DNA collected from the crime scene and was a perfect match, which proved that he was present when the couple was murdered.

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