Manuela Allen went to bed in July 2019, but by the morning, she was gone. Blood splatters in her bedroom suggested the teacher had been the victim of a brutal crime.

Eugene Mallove was a beloved family man and an accomplished scientist who had come down to Norwich, Connecticut to clean a home he'd been renting out when he was viciously murdered.

It would take more than three decades for investigators to learn who shot Barbara Rocky, a 1974 Brigham Young University student, to death in Big Cottonwood Canyon. 

Patricia Kimmi vanished in November 2009, but investigators suspected foul play after discovering her dentures, a money clip covered in blood, and bloody coins not far from her Kansas home.

"My whole world changed," Laurie Stout's sister said of the moment she learned her big sister was dead. "I no longer had my confidante and no longer had my best friend. It was horrible."

Dennis and Merna Koula had been devoted grandparents and parents when they were gunned down in their quiet Wisconsin community in May of 2010. 

Joseph Wellnitz, his wife, Beth, and 20-year-old son Dennis were all found shot to death in the family's Columbia, Kentucky home in 1993, but it would take investigators nearly a decade to find the surprising killer.

Omar Hernandez, a married father of two, was leaving for the graveyard shift at a local grocery store on October 9, 2016 when he was suddenly ambushed outside his home and shot.

Samantha Lezark went looking for love in online chat rooms in 2003, but did her desire to find a new partner cost her life?

"An Unexpected Killer" returns to Oxygen with all new episodes, including one about the 1979 murder of Savannah Anderson.