Dateline: Secrets In Pleasant Prairie - 5 harrowing details about Julie Jensen's murder, explored

True-crime series Dateline tackles the harrowing murder of Julie Jensen in Season 32, Episode 4, titled Secrets in Pleasant Prairie. The episode originally aired on October 12, 2023, and was re-aired on April 19, 2025. Julie Jensen's chilling murder in her home by her own family was also featured on 20/20, titled The Kill List, on February 10, 2023.

Jensen wrote a letter preempting her murder after suspicions about her husband, and this letter was pivotal in this case as it led to two trials over 25 years, as well as a conviction for her murderer.

Here are 5 harrowing details about Jensen's murder, as featured on Dateline.

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5 Harrowing Details about Dateline's Secrets in Pleasant Prairie

Julie Jensen preempted her murder in a letter

According to the Dateline episode, Julie Jensen was a 40-year-old wife with two kids. On December 3, 1998, she was found dead in her home in Prairie. The investigation of her murder started, but it took an extremely surprising turn when the police found a letter that Julie Jensen had written on November 21, 1998. In this letter, she wrote:

"If anything happens to me, he would be my first suspect."

The 'he' that Julie refers to is her husband Mark. Julie preempted her murder and wrote this letter in case anything happened to her. She gave this letter to her neighbours, Margaret and Ted Wojt, asking them to hand over the letter to the police if anything happens to her.

The police found this letter a day after her death, and were shocked at what they found in the letter. Dateline revealed that Julie detailed in her letter that she was scared of her husband, afraid that he had never forgiven her for her affair seven years ago. She explained that her husband was behaving suspiciously, urging her to drink more. She also included a shopping list that she found, belonging to her husband. This list included items like syringes and chemicals.

Her husband's behavior and the suspicious shopping list led her to believe that her life was in danger. So, she wrote a letter, which played a huge role in her husband's conviction, after being admitted as evidence. The letter was later excluded from evidence in the 2023 retrial of the case, but it was greatly responsible for Mark's conviction.

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Julie was right to be suspicious of the shopping list

The letter that Dateline described included a shopping list that Julie discovered. She said that the list had suspicious items such as chemicals and syringes. Turns out, Mark Jensen had been using ethylene glycol, a chemical that is the main ingredient in antifreeze, to poison his wife slowly.

Julie's autopsy and toxicology reports after her death revealed that her stomach, urine, and blood contained traces of antifreeze. Prosecutors revealed that Mark must have been slowly killing his wife by mixing the chemical in her food or drinks (he was urging her to drink more). She had been complaining of weakness and breathing issues for days.

On December 3, he administered sleeping pills to weaken her even more, and then went on to suffocate her to death with a pillow while she was barely conscious.

Dateline also revealed that their home computer had an internet search history related to ethylene glycol poisoning and toxicology, further linking Mark to his wife's death and proving that his crime was methodical and premeditated.

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Julie was not the only one who had an affair

Dateline revealed that even though Julie thought that Mark's motive for murdering her was the fact that she had cheated on him years ago, this was not the case. According to WSN reports, Mark Jensen also had an affair with Kelly LaBonte, his married co-worker, before Julie's death.

Mark's devices revealed emails to her expressing his love and affection for her, and promising a new life to her, where they could be together. Prosecutors argued that Jensen wanted to start a new life with LaBonte, which is why he chose to kill his wife rather than attempt to divorce her. Jensen went on to marry Kelly after Julie died, and she had custody of their two kids after Mark was imprisoned.

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It took two trials to cement Mark's conviction

Dateline reported that Mark Jensen was originally convicted on February 22, 2008, after a trial in Walworth County that spanned six weeks. His sentence- prison for life without parole- relied heavily on Julie's letter, but in 2016, this conviction was overturned by a federal judge. The judge cited a violation of Mark's right to confront his accuser. However, the Wisconsin Supreme Court ordered a retrial in 2021, excluding the voicemails and the letter from evidence.

The second trial in 2023 also ended with a jury convicting Mark after deliberating for a day, resulting in Mark being sentenced to life without parole.

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An informant's statement helped cement Mark's fate

Even though Julie's letter, the toxicology report, and the shopping list pointed heavily to Mark being guilty, it was the confession of an informant in the Kenshoa County Jail that cemented Mark's fate. Allegedly, Mark confessed to this inmate specific details about how Julie's murder was committed, which, according to the prosecutors, only the perpetrator would know.

The inmate's confession sealed the case in the 2008 trial and was also considered in the 2023 retrial. Defense claimed that the informant was only looking for a lighter sentence, but the confession, paired with forensic evidence, helped Mark's conviction in the retrial.

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According to Dateline: Secrets in Pleasant Prairie, Mark Jensen, now 63, serves his time at Oshkosh Correctional Institute.

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Keep reading Soap Central for more such harrowing Dateline cases.