(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.