Deadly Betrayal: The Antifreeze Murder That Took 25 Years to Solve
On the morning of December 3, 1998, Mark Jensen made a chilling discovery—his wife, Julie Jensen, lay lifeless in their bed. He told investigators she had been sick for days, but beyond that, he had no answers. What seemed like a tragic, unexplained death would soon unravel into a twisted tale of deception, revenge, and cold-blooded murder.
The initial autopsy yielded no clear cause of death, leaving the authorities perplexed. But when detectives dug deeper—especially into the Jensens’ home computer—they stumbled upon a digital trail of deadly intentions. What followed was a legal battle spanning more than two decades, culminating in two murder trials and a dramatic conviction.
A Marriage Tainted by Secrets
Mark and Julie Jensen appeared to be living a quiet life in Pleasant Prairie, Wisconsin, a small town on the shores of Lake Michigan. Mark worked as a stockbroker, while Julie cared for their two young sons. But behind closed doors, their marriage was anything but picture-perfect.
Years before her death, Julie had a brief affair with a coworker, a mistake she confessed to Mark. After that, she became the target of relentless harassment—pornographic photos left on their property, anonymous phone calls, a campaign of psychological torment. She meticulously documented these incidents, but the culprit remained unknown.
By late 1998, Julie’s mental health had taken a hit. On December 1, just two days before her death, she visited her doctor, who prescribed her Paxil, an antidepressant. This, Mark Jensen’s defense team later argued, was evidence that she had taken her own life. But prosecutors had a far darker theory.
A Killer’s Digital Footprint
Authorities seized the Jensens’ home computer and found incriminating searches detailing ways to kill someone—specifically, ethylene glycol poisoning. The key ingredient in antifreeze, ethylene glycol is deceptively lethal. It tastes sweet, making it easy to slip into a drink unnoticed. Once ingested, the symptoms mimic severe flu or food poisoning, leaving victims vulnerable before organ failure sets in.
Julie Jensen’s toxicology report confirmed it: her blood, stomach, and urine contained ethylene glycol. Investigators suspected that Mark had first drugged her with sleeping pills before poisoning her. As her body weakened, he allegedly finished the job by suffocating her.
The timing of Julie’s death raised further suspicions. In the months leading up to it, Mark had been having an affair with a woman named Kelly LaBonte. Just weeks after Julie was buried, LaBonte moved into the Jensens’ home. Eventually, the two married. Prosecutors argued that Julie’s death had been a calculated move to clear the path for his new lover.
A Chilling Letter from the Grave
But the most damning piece of evidence wasn’t digital—it was written by Julie herself.
Days before her death, Julie gave a sealed letter to a neighbor with strict instructions: if anything happened to her, hand it over to the police. The message inside was haunting:
“If anything happens to me, [Mark Jensen] would be my first suspect.”
She also emphasized that she would never take her own life because of her children—they were everything to her.
Inside the envelope was another disturbing clue: a photograph of a handwritten list Mark had made, including cryptic phrases like “own drug supply,” “bag hands,” and “syringe.” To investigators, this was no ordinary to-do list—it was a murder blueprint.
A Courtroom War: Two Trials, One Verdict
In 2008, nearly a decade after Julie’s death, Mark Jensen was convicted of first-degree murder and sentenced to life in prison. The prosecution had presented Julie’s letter as crucial evidence. But Jensen fought back, arguing that he never had the chance to confront his accuser—his dead wife—and that her letter should have been inadmissible.
The case dragged on for years, making its way to the Wisconsin Supreme Court. In 2021, Jensen won a retrial on the grounds that the letter violated his constitutional rights. Prosecutors had to rebuild their case without it.
Even without Julie’s posthumous warning, the evidence was damning. The poison, the internet searches, the affair, the rapid replacement of his wife—it all pointed in one direction. In January 2023, Mark Jensen faced a jury once again. They found him guilty, and the judge handed down the maximum sentence: life in prison without parole.
A Crime Too Monstrous to Forgive
At his sentencing, Judge Bruce Schroeder delivered a final, damning statement:
“Your crime is so enormous, so monstrous, so unspeakably cruel, that it overcomes all other considerations.”
Mark Jensen now sits behind bars at Oshkosh Correctional Institute, serving out his life sentence.
For Julie Jensen, justice took decades—but in the end, the truth prevailed. And in a case where a murderer thought he had erased his wife’s voice, it was her own words, written from beyond the grave, that ultimately sealed his fate.
