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Dateline episode Secrets in Pleasant Prairie, which explores the mysterious death of Julie Jensen, premiered today on Oxygen. On December 3, 1998, 40-year-old Julie Jensen was found dead in her bed in Pleasant Prairie, Wisconsin.
Her death was initially thought to be from natural causes since there were no obvious signs of trauma, and her husband, Mark Jensen, told investigators she had been ill and depressed in the days leading up to her death.
However, when investigations into her death began, authorities began to suspect foul play, and the case evolved into one of Wisconsin’s most infamous murder cases, involving antifreeze poisoning, a chilling letter from the victim predicting her own demise, and two separate murder trials nearly 25 years apart.
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Who was Julie Jensen? Relationship with Mark Jensen explored
Julie Jensen was one of six siblings who grew up in Wisconsin. While working at Sears, she met her husband, Mark Jensen, and the two dated throughout college and married in 1984. They eventually settled in Pleasant Prairie and had two sons, David and Douglas. Mark was a successful stockbroker, while Julie stayed home to raise their children after briefly working in finance herself.
However, beneath the surface of their stable marriage, there was trouble. Julie had once confessed to a brief affair with a co-worker, and although it had ended years earlier, Mark never forgave her. By the late 1990s, Mark himself started having an affair with a colleague, Kelly LaBonte, who would move into the Jensen home just weeks after Julie’s death.
A crucial piece of evidence
As reported by ABC, in the weeks preceding her death, Julie Jensen confided her fears to friends and neighbors. She had discovered troubling notes in her husband’s datebook, including references to “drug supply” and “syringe.” Believing her life was in danger, she wrote a letter addressed to the police and entrusted it to her neighbors, Ted and Margaret Wojt.
Julie wrote that if anything happened to her, Mark should be considered her “first suspect.” She also added that she would never take her own life because of her children. This letter would become one of the most contested pieces of evidence in the entire case.
Toxicology reports and other evidence
Initially, toxicology reports showed nothing unusual, but further testing revealed ethylene glycol, the toxic ingredient in antifreeze, in Julie’s system. Evidence suggested the poison had been administered gradually, and investigators also found that someone in the household had searched online for the effects of ethylene glycol poisoning on the morning of Julie’s death.
Prosecutors alleged Mark administered antifreeze to his wife over several days, then suffocated her with a pillow, as per Oxygen. Mark Jensen was charged with first-degree intentional homicide in 2002. In his 2008 trial, Julie’s letter was used as a central piece of evidence.
Prosecutors also presented testimony from jailhouse informant Aaron Dillard, who claimed Mark admitted to poisoning Julie Jensen and later smothering her. Another colleague, Ed Klug, recalled Mark musing about poisoning methods shortly before Julie’s death. The jury convicted Mark, and he was sentenced to life without parole.
Mark's trial and sentencing
However, in 2013, a federal judge overturned Mark's conviction, ruling that Julie’s letter violated Mark’s Sixth Amendment right to confront witnesses. In 2023, prosecutors retried Mark Jensen without the letter, as per ABC.
They emphasized computer forensics linking Mark to years of harassment against his wife, including planting pornographic images to torment her. Prosecutors also highlighted internet searches on ethylene glycol poisoning conducted from the family computer when Julie was too incapacitated to perform them herself.
Even without Julie’s letter, the circumstantial evidence was powerful enough to convince the jury. On February 1, 2023, a jury again found Mark Jensen guilty of first-degree intentional homicide, and he was sentenced once more to life in prison without parole.
Julie Jensen's case has been explored on Oxygen.
