In 1992, Peterson married Kathleen Savio, an accountant, just two months after he finalized his divorce from his second wife, Vicki Connolly. The couple had two sons together during their union. They split in 2001, when Savio learned that her police officer husband was having an affair with a village employee.

Their divorce wasn't finalized until 2003 and, according to Chicago Magazine, the authorities were called "repeatedly" to intervene between Peterson and Savio.

In March of 2004, Savio was found dead in the bathtub located in the primary bathroom of her home. Though the tub did not have water in it, Savio's hair was wet. There were also bruises and abrasions on her body, and there was a gash in her scalp. About two months later, her death was ruled to be accidental.

The case was reopened in 2007 following the disappearance of Stacy Ann Cales. Upon a further investigation, it was determined that Savio had drowned following a struggle.

In 2009, Peterson was arrested and indicted for the premeditated murder of Kathleen Savio. He was found guilty in September 2012, and he was sentenced to 38 years in prison the following year.