DNA tests link Gillis to the murder of thirty-year-old Katherine Hall. Hall was a drug addict who spent a lot of time on the street. It was somewhere around North Street where Sean Vincent Gillis found her on a cold night in January 1999. She got into his car. Later, he slipped a plastic cable tie around her neck and choked her. He then stabbed her in the throat and through her left eye.

In May 1999, Gillis began stalking Hardee Schmidt, a 52-year-old mother of three who had once competed in the Boston Marathon. About 3 weeks later, on Sunday, May 30, Gillis spotted Hardee jogging and hit her with his car, knocking his victim into a ditch. He jumped out and looped a heavy-duty cable tie around the stunned woman's neck and started choking her. Gillis then dragged Hardee into his car and drove to a nearby park where he raped and murdered her, mutilated her with a knife, then stuffed her body into the trunk of his car. The next day, Gillis drove south on Airline Highway toward New Orleans. In St. James Parish, 35 miles from Baton Rouge, he dumped Hardee's body in a small bayou that flowed alongside the highway. He has confessed to this gruesome murder.

On November 12, 1999, Gillis picked up 36-year-old Joyce Williams shortly after she left a house on Oriole Street in Baton Rouge. He drove her across the Mississippi River and killed her. Hunters found her skeletal remains two months later lying on the bank of the river in Iberville Parish. One of her legs had been severed. Gillis confessed to the murder.