On December 3, 1998, Mark Jensen found the body of his wife, Julie Jensen, in their bed.
He told investigators she’d been sick for the past three days, but that, otherwise, he didn’t know what had happened to her.
The initial autopsy was inconclusive, with Kenosha County, Wisconsin, medical examiners unable to determine the cause or manner of 40-year-old Julie Jensen’s death. But after investigators seized the family’s computer, they began to suspect Mark Jensen had murdered his wife.
In the end, it took two murder trials—one in 2008 and another in January 2023—to convict Mark Jensen of killing his wife by poisoning her with antifreeze, drugging her with sleeping pills and, eventually, suffocating her.
After years of legal back and forth—which centered largely on a letter Julie Jensen had written shortly before her death pointing the finger at her husband should anything happen to her—a judge sentenced Mark Jensen to life in prison without the possibility of parole.
The case against Mark Jensen was the subject of an episode of A&E’s Killer Cases. [Stream episodes of Killer Cases in the A&E app now.]
Suicide or Murder?
Mark and Julie Jensen lived in Pleasant Prairie, Wisconsin, a small town situated on the shores of Lake Michigan about 40 miles south of Milwaukee. Mark Jensen worked as a stockbroker, while Julie Jensen stayed home and watched the couple’s two young sons, David and Douglas.
The Jensens’ marriage was a rocky one. In the early 1990s, Julie Jensen had a brief affair with a coworker and, after she admitted the infidelity to Mark, someone began harassing her. For years, the unknown perpetrator left pornographic photos around the house and on the Jensens’ vehicles and made hang-up phone calls, which Julie diligently recorded in a logbook.
Julie Jensen’s mental health suffered and, on December 1, 1998, just two days before her death, she visited the family doctor, who wrote her a prescription for the antidepressant Paxil.
“She was miserable and depressed,” the doctor, Richard Borman, testified during the January 2023 trial.
At trial, Mark Jensen’s attorneys used Julie Jensen’s mental health struggles as part of their defense, arguing that she had died by suicide and framed her husband for murder.
But prosecutors painted a very different picture.
When they scoured the internet browser history on the family’s computer, they found searches for ways to kill someone, including ethylene glycol poisoning. Ethylene glycol is a primary ingredient of antifreeze.
Medical examiners also found ethylene glycol in Julie Jensen’s blood, stomach and urine.
“Antifreeze is very, very available and it is very bad for you,” Jennifer Shen, a retired crime lab director for the San Diego Police Department who was not involved with the case, tells A&E True Crime. “If you ingest it, it can be a long time before you even realize something is severely wrong with you. But it would be difficult to explain how an adult would ingest a lot of antifreeze.”
Investigators also found emails that indicated Mark Jensen had been having an affair with a woman named Kelly LaBonte (now Kelly Brooks) in the months leading up to Julie Jensen’s death. Not long after Julie Jensen died, LaBonte moved in with Mark Jensen and, a few years later, the two were married.
Prosecutors argued that Mark Jensen killed his wife so he could be with LaBonte. They also claimed he had never forgiven his wife for the brief affair she had with a coworker.
“Mark Jensen, who had tormented and punished and never forgiven his wife for years, found her replacement and then killed her,” said Carli McNeill, Kenosha County deputy district attorney, during closing arguments of the January 2023 trial.
If Mark Jensen did kill his wife so that he could be with his new lover, poison makes sense as a murder weapon, says Shen. He likely didn’t love his wife anymore, so he wouldn’t have cared if she was in pain. And poison suggests he spent time carefully planning how to get rid of her.
“In a lot of murders, someone just flips out and stabs or shoots someone—there’s a situation that pushes them to have a very violent, physical response,” she says. “Whereas poisoning is more deliberate and planned out. It’s generally more premeditated, and it’s usually a more dispassionate approach. You’re watching that person suffer and you’re fine with it because it’s a means to an end. You’re not connected to your victim anymore.”
The Letter From the Grave
Beyond Mark Jensen’s extramarital affair and the incriminating internet search history they found on the computer, investigators were also convinced of his guilt by a letter Julie Jensen had written just before her death.
That letter, however, also created years of legal headaches.
About a week and a half before she died, Julie Jensen penned a letter, sealed it inside an envelope and gave it to a neighbor. In it, she wrote that “if anything happens to me, [Mark Jensen] would be my first suspect.”
According to the Killer Cases episode, she also wrote: “I would never take my life because of my kids—they are everything to me!”
Inside the envelope, she also included a photo she had taken of a list written in Mark Jensen’s handwriting that included items like “own drug supply,” “bag hands” and “syringe.”
“It certainly seemed Mark Jensen was making a list of potential ways to murder someone,” says prosecutor Angelina Gabriele in Killer Cases.
Letters like this are rare, but not unheard of, in murder cases, says Shen. And, if the Jensens’ marriage was falling apart, it’s not surprising that Julie Jensen felt some “indication that something was not right,” she says.
Prosecutors presented Julie Jensen’s so-called “letter from the grave” to jurors during Mark Jensen’s first murder trial in 2008. At the end of that trial, the jury convicted Mark Jensen of killing his wife, and a judge sentenced him to life in prison without parole.
But Mark Jensen argued that the letter should not have been admissible in court because it violated his constitutional right to cross-examine a witness who provided evidence against him. Since Julie Jensen was dead, Mark Jensen couldn’t confront her and, therefore, the letter shouldn’t have been allowed at trial, he argued.
The case made it all the way to the Wisconsin Supreme Court, which in 2021 ruled that Mark Jensen deserved a new trial—one at which the letter would not be allowed as evidence.
In January 2023, prosecutors tried the case for a second time and again secured a conviction. Judge Bruce Schroeder handed down the maximum sentence in the case: life in prison without parole.
Mark Jensen is currently incarcerated at Oshkosh Correctional Institute in Oshkosh, Wisconsin.
“Your crime is so enormous, so monstrous, so unspeakably cruel, that it overcomes all other considerations,” Schroeder said at sentencing.
Related Features:
Toxic Endings: When Poison is the Murder Weapon
How Diane and Rachel Staudte Killed Their Family With Antifreeze
Guns, Knives, Hands: Why Killers Choose Specific Murder Methods
