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The new Dateline episode Secrets in Pleasant Prairie, revisits the chilling case of Julie Jensen, a 40-year-old mother of two who was found dead in her Pleasant Prairie, Wisconsin, home on December 3, 1998.
At first, her death appeared natural as there were no visible injuries, and her husband, Mark Jensen, claimed she had been sick and depressed. But as investigators dug deeper, suspicion mounted, uncovering evidence of antifreeze poisoning and a haunting letter Julie Jensen wrote naming her husband as her potential killer. The episode aired today, September 26, 2025, on Oxygen.
Julie Jensen's case: 5 details explored
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1. The tragic discovery
40-year-old Julie Jensen was found dead in her Pleasant Prairie, Wisconsin, home by her husband, Mark Jensen, on December 3, 1998. Since there were no obvious signs of trauma and Mark told investigators that Julie had been ill, her death was considered due to natural causes.
However, investigators started noting inconsistencies in the case, and initially, what appeared to be a natural death soon unraveled into one of Wisconsin’s most notorious murder cases.
2. A troubled marriage
Julie Jensen was a Wisconsin native, one of six children, and remembered as a devoted mother to her two sons, David and Douglas. She met Mark Jensen while working at Sears, and the two married in 1984 after dating through college. They settled into Pleasant Prairie, where Mark worked as a stockbroker, and Julie focused on raising their family after briefly working in finance.
Outwardly, their marriage looked stable, but under the surface, there was trouble. Julie had once admitted to a brief affair with a co-worker, a mistake that Mark never forgave. But by the late 1990s, Mark himself was engaged in an affair with a colleague, Kelly LaBonte, who would move into the Jensen home shortly after Julie’s death.
3. A letter from Julie Jensen
One of the most haunting elements of the case was a letter Julie wrote just days before her death. She was disturbed by ominous notes she had discovered in Mark’s datebook, which included phrases like “drug supply” and “syringe”, and she confided in friends and neighbors that she feared for her life.
Julie entrusted a letter to neighbors Ted and Margaret Wojt with instructions to give it to the police if anything ever happened to her. Inside was a letter addressed to authorities in which Julie stated that if she were ever harmed, Mark should be considered her “first suspect.” She added that she would never take her own life because of her children. As per Oxygen, the letter was one of the most crucial and contested pieces of evidence in court.
4. Mark's first trial
An initial toxicology report found nothing unusual, but later investigations revealed ethylene glycol, the lethal ingredient in antifreeze, in Julie’s system. The poisoning appeared to have occurred over several days. Investigators also uncovered internet searches from the Jensen family computer on the effects of ethylene glycol poisoning, which was conducted while Julie was too incapacitated to operate the computer herself.
Prosecutors alleged that Mark had poisoned Julie and, when the antifreeze acted too slowly, smothered her with a pillow. In 2002, Mark was charged with first-degree intentional homicide.
At his 2008 trial, prosecutors presented Julie’s letter, testimony from jailhouse informant Aaron Dillard, who claimed Mark confessed to poisoning and smothering his wife, and testimony from Mark’s co-worker, Ed Klug, who recalled Mark speculating about poisoning. The jury convicted Mark, and he was sentenced to life without parole.
5. Mark's retrial
In 2013, a federal judge overturned Mark’s conviction, ruling that Julie’s letter violated his Sixth Amendment right to confront witnesses. A retrial was ordered without the letter involved, and in January 2023, Mark Jensen faced the court again, as reported by ABC.
Prosecutors this time leaned on digital evidence, including proof that Mark himself had planted pornographic images to harass Julie and had conducted searches about antifreeze poisoning on the day she died. Even without Julie’s letter, the circumstantial evidence convinced jurors. On February 1, 2023, Mark Jensen was once again found guilty of first-degree intentional homicide and sentenced to life without parole.
Julie Jensen's case is explored on the Oxygen episode Secrets in Pleasant Prairie.
