Julie Jensen’s case – An in depth case overview

The Dateline episode Secrets in Pleasant Prairie, which premiered today on Oxygen, revisits the haunting case of Julie Jensen, a 40-year-old mother of two found dead in her Pleasant Prairie, Wisconsin, home on December 3, 1998.

At first, her death appeared natural, with no visible injuries, and her husband, Mark Jensen, claimed she had been ill and depressed. But as investigators probed deeper, suspicion grew. Toxicology revealed antifreeze poisoning, and a disturbing letter Julie wrote before her death named her husband as her “first suspect.” What followed was one of Wisconsin’s most infamous murder cases, spanning two trials.

Julie Jensen’s death explored

Julie Griffin was one of six siblings from Wisconsin who met Mark Jensen while at Sears. The pair dated through college and married in 1984, settling in Pleasant Prairie and starting a family. Julie left her career in finance to raise their sons, David and Douglas, while Mark advanced as a stockbroker.

While their life seemed stable, there were cracks in the marriage. Julie Jensen had confessed to a brief affair, which Mark never moved past. Soon, in the late 1990s, Mark himself was involved in an affair with colleague Kelly LaBonte.

As per ABC, Julie started voicing concerns about her safety to her friends and family when she noticed disturbing entries in Mark’s datebook, such as “drug supply” and “syringe.” Believing her husband was plotting against her, Julie wrote a letter to the police detailing her suspicions and entrusted it to her neighbors, Ted and Margaret Wojt.

In the letter, she specified that if anything happened to her, Mark should be considered her “first suspect.” She also emphasized she would never harm herself, as she wanted to be there for her children.

Mark Jensen emerged as the suspect

On the afternoon of December 3, 1998, Mark Jensen called authorities after finding Julie Jensen unresponsive in bed. There were no obvious signs of trauma, and Mark told investigators that Julie had been sick for days. Initially, her death was considered natural, but authorities soon noticed inconsistencies in the case.

The initial toxicology tests revealed nothing suspicious, but further analysis detected ethylene glycol, the lethal ingredient in antifreeze, in Julie’s system. Experts concluded the poison had been ingested gradually over several days.

They also discovered computer searches about ethylene glycol poisoning conducted from the Jensen household on the morning of Julie’s death, when Julie was reportedly too incapacitated to use the computer.

In 2002, prosecutors charged Mark Jensen with first-degree intentional homicide. They alleged he poisoned his wife with antifreeze and smothered her with a pillow when the poison acted too slowly, according to Oxygen.

Mark’s first trial took place a decade after Julie’s death, in 2008. Prosecutors introduced Julie’s letter, her neighbors’ testimony, computer evidence, and witness accounts. A jailhouse informant, Aaron Dillard, testified that Mark admitted to poisoning and later suffocating Julie Jensen.

Former co-worker Ed Klug also recalled Mark discussing methods of poisoning shortly before Julie’s death. Accoridng to ABC, the jury found Mark Jensen guilty, sentencing him to life in prison without parole for first-degree intentional homicide.

Mark’s retrial explored

Five years later, in 2013, a federal judge overturned Mark’s conviction. The court ruled Julie’s letter violated Mark’s Sixth Amendment right to confront witnesses since she could not be cross-examined, and a retrial was ordered.

Nearly 25 years after Julie’s death, Mark was retried. This time, prosecutors did not rely on Julie’s letter. Instead, they emphasized digital forensics, including proof that Mark harassed Julie by planting pornographic materials in the computer.

They also pointed to the antifreeze searches made on the family computer when Julie was too incapacitated to do it herself. Although the letter was excluded, the circumstantial evidence was compelling. As per Oxygen, on February 1, 2023, a jury once again convicted Mark Jensen of first-degree intentional homicide, sentencing him to life in prison without parole in April 2023.

Catch the case of Julie Jensen on Oxygen.