Deadly Betrayal: The Antifreeze Murder That Took 25 Years to Solve
When Mark Jensen called authorities on December 3, 1998, claiming his wife had died suddenly in their Pleasant Prairie home, it appeared to be a tragic case of illness gone wrong. Julie Jensen, just 40 years old, was found lifeless in bed. But this wasn’t a story of natural causes. It would take two trials, two decades, and one damning letter from beyond the grave to expose the truth behind the Julie Jensen antifreeze murder.
The Mask of a Suburban Marriage
To outsiders, the Jensens were the picture of stability. Mark worked as a stockbroker, Julie stayed home with their two sons, and life in Wisconsin seemed quiet. But the facade hid growing tension and deep betrayal.
Years earlier, Julie had admitted to a brief affair. From that point on, she claimed, her life became a nightmare. She documented strange incidents: disturbing pornographic images left around their home, frightening phone calls, and an overwhelming sense of being watched. Julie suspected Mark.
Despite it all, she focused on raising her boys. But by December 1998, her health had started to decline. Her doctor prescribed an antidepressant. When she died two days later, Mark suggested suicide. Authorities weren’t so sure.
Poison in Plain Sight
Julie’s autopsy raised more questions than it answered—until toxicology results came in. The findings were startling: her system contained ethylene glycol, the main component of antifreeze.
What made the discovery chilling wasn’t just the poison—but how it was delivered. Investigators believed Mark had slipped it into her drinks, possibly masking the taste with juice. Ethylene glycol is sweet, easy to miss, and deadly once metabolized. It slowly destroys internal organs while mimicking flu-like symptoms.
Adding to suspicion, authorities uncovered incriminating internet searches on the Jensen family computer—detailed instructions on how to poison with antifreeze. This, prosecutors argued, was no accident. It was premeditated murder.
A Letter That Refused to Stay Silent
The turning point in the Julie Jensen antifreeze murder wasn’t digital—it was deeply personal. Before her death, Julie handed a sealed envelope to a trusted neighbor with a chilling message: give this to the police if anything ever happened to her.
Inside was a handwritten letter. Her words were stark:
“If anything happens to me, Mark Jensen would be my first suspect.”
She made it clear—she loved her children too much to take her own life. Alongside the letter was a photograph of a strange handwritten list allegedly penned by Mark, with phrases like “drug supply,” “bag hands,” and “syringe.” Investigators called it a murder checklist.
The Other Woman and the Motive
Mark Jensen’s actions following his wife’s death fueled the prosecution’s theory. Within weeks, his girlfriend, Kelly LaBonte, moved into the Jensen family home. Eventually, they married. Prosecutors claimed this was Mark’s motive: remove Julie to make room for a new life.
The narrative of a calculated plan—slowly poison his wife, then quickly replace her—was reinforced by his behavior and the medical timeline.
Courtroom Battles and Constitutional Rights
In 2008, a jury found Mark Jensen guilty of first-degree murder. The letter from Julie, written just before her death, had sealed the case. He was sentenced to life without parole.
But the legal saga didn’t end there.
Mark’s defense argued that the letter violated his Sixth Amendment right to confront his accuser. Julie was no longer alive to be cross-examined. After years of appeals, the Wisconsin Supreme Court agreed. A retrial was granted in 2021—this time, without Julie’s letter as evidence.
Still, prosecutors pressed on. Even without her words, the facts remained damning: the toxicology, the internet searches, the affair, and his behavior after her death.
In January 2023, nearly 25 years after Julie’s murder, Mark Jensen was convicted again.
Justice, Finally Served
At sentencing, Judge Bruce Schroeder didn’t mince words:
“Your crime is so enormous, so monstrous, so unspeakably cruel, that it overcomes all other considerations.”
Mark Jensen will spend the rest of his life in prison at Oshkosh Correctional Institution, with no chance for parole.
A Voice That Refused to Be Silenced
Julie Jensen’s death shocked her community, but her voice—written in careful, fearful words—helped unravel her husband’s deadly plan. She protected her children in life, and in death, she exposed the truth.
The Julie Jensen antifreeze murder case stands as a chilling reminder of how justice can take time, but persistence, evidence, and a mother’s love can outlast even the darkest betrayal.
FAQs
What was the cause of Julie Jensen’s death?
Julie died from ethylene glycol poisoning, the active ingredient in antifreeze, confirmed by toxicology.
Was Julie’s letter used in both trials?
No. It was used in the 2008 trial but ruled inadmissible in the 2023 retrial due to constitutional concerns.
Why did the case take so long to resolve?
Appeals, legal debates over admissible evidence, and retrials prolonged the case for nearly 25 years.
What role did the internet search history play in the case?
Searches for antifreeze poisoning methods on the family computer helped prove premeditation.
Who is Kelly LaBonte?
Mark Jensen’s girlfriend and later wife, who moved into his home shortly after Julie’s death.
What was Mark Jensen’s final sentence?
He received life in prison without the possibility of parole.