A Missouri man is set for execution Tuesday for killing his girlfriend and her young daughter during a violent argument.

Defense attorney Jennifer Herndon said Richard Strong "just snapped" during a heated exchange between two people with mental health issues. He used a butcher's knife to kill Eva Washington, then turned the weapon on Washington's 2-year-old daughter from a previous relationship.

Police found the bodies on Oct. 23, 2000, at Washington's apartment in suburban St. Louis. They also found a pool of blood on a bed, the knife next to it, and the couple's 3-month-old daughter, Alyshia Strong, who was unhurt.

Alyshia Strong is now 14 and pleaded for her father's life in a clemency petition to Gov. Jay Nixon. The governor is still weighing that request.