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Murder of Theresa Wesolowski by Mark Libecki featured on The Killer Beside Me

Theresa Wesolowski and Mark Libecki photo
Theresa Wesolowski was murdered by her co-worker, Mark Libecki. Pic credit: Maryland Department of Corrections / Family

The Killer Beside Me on Investigation Discovery delves deeper into the case of 22-year-old Theresa Wesolowski, who was murdered by her co-worker, Mark Libecki.

On May 28, 1999, Wesolowski was found dead next to her vehicle, which was parked about a half-block from Stone Container — a box factory where she worked — in Germantown, Wisconsin. She had been stabbed more than 40 times.

Police officials suspected Wesolowski’s boyfriend or her ex-boyfriend of murdering her until 2006 when they ascertained that neither of their DNA matched the that found on her body.

When investigators asked several people who were close to the victim to provide them with DNA samples, they came back with a match. With new technology, police found Libecki’s DNA on Wesolowski’s hands and his shoe print was found in her blood.

Despite having that information, Libecki was not arrested until 2009 when police discovered Wesolowski’s blood inside the Ford Explorer Libecki owned in 1999.

Libecki denied killing Wesolowski. He testified that on the night of the murder, around 11 pm, another co-worker named Tommy J. Thompson, who died of an overdose in 2003, met up with him at a location near his job to buy cocaine.

A short while later, Wesolowski arrived and joined Thompson in the backseat of his vehicle and began discussing something, but Libecki said he wasn’t sure what they were saying, and he said he didn’t want to know.

Libecki went on to say that after Thompson snorted the cocaine, he zoned out and before he knew it, Thompson was attacking Wesolowski in the backseat.

That’s when Libecki said he got out of the vehicle and opened the back door in an effort to get them out of his car, but it was at that time he saw blood and Wesolowski was slumped over.

He said he dragged her out of the car and put her body on the street, next to her vehicle. Before fleeing the scene, Libecki claimed that Thompson told him if he ever told anyone about what happened, he would kill him.

However, investigators weren’t buying his story as DNA evidence linked him to the murder. Libecki was arrested and charged with first-degree intentional homicide as a party to the crime.

He was found guilty of murdering Wesolowski and was sentenced to life in prison without the possibility of parole.

The Killer Beside Me — Roadside Murder, airs at 9/8c on Investigation Discovery.

Angelica N. Sumter is a freelance journalist who has covered true crime for more than 10 years. In addition to Monsters and Critics, her work... read more
Angelica N. Sumter


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