(NBC News) — Thursday on an all-new “Dateline,” the investigation into the poisoning death of Julie Jensen, a Wisconsin mother of two, takes a surprising turn when detectives learn she wrote a letter just days before her death pointing to a possible suspect.

Here’s a preview of Andrea Canning’s report:

Bob Jambois, the Kenosha County district attorney at the time rushed over to the Jensen home.

BOB JAMBOIS: And as soon as I saw Julie Jensen’s body, I could observe that the positioning of her body was — the way her arm was spread out underneath her. Um, it — I said she was rolled into that position. Nobody, nobody, goes in that position naturally of their own accord.

Investigators began gathering evidence and noticed the family had a home computer — in 1998 they were still considered a luxury.

BOB JAMBOIS: And I said, we’re going to be taking that computer. This is the first time I’d actually seen a computer in the home of — at a crime scene.

As they continued to search, investigators learned something potentially crucial to the case. It turns out, local police were quite familiar with Mark’s wife.

BARRY OLLILA: Our officers had a relationship with Julie Jensen. They knew each other on a first-name basis based on the number of calls that she, she, placed at our agency.

Julie had been calling the Pleasant Prairie Police Department for years to report harassment: Repeated hang-up calls to her and her husband.

And even more frightening, she’d said they found pornographic pictures planted at Mark’s office and outside their house.

ANDREA CANNING: Your prosecutor’s senses, kind of, must be going off given that the police have been there so many times.

BOB JAMBOIS: Yeah. Prosecutors don’t believe in coincidences.

Watch “Dateline: Secrets in Pleasant Prairie,” Thursday at 10 p.m. on NBC4.

About ‘Dateline’

“Dateline NBC” is the longest-running series in NBC primetime history and is in its 32nd season. Dateline is anchored by Lester Holt and features correspondents Andrea Canning, Josh Mankiewicz, Keith Morrison and Dennis Murphy.

The stories range from compelling mysteries to powerful documentaries and in-depth investigations. When major news breaks, they go to the scene, putting the pieces together to bring the viewer the full picture. And in every story they tell, they help the real people who lived the events share their journeys with the viewer.