Keith Burley, 43, was involved with a domestic incident in Union Township around 10:30 p.m. on Monday, according to Pennsylvania State Police.
Authorities said Burley assaulted a 36-year-old woman and later drove away in her vehicle — along with her two children, ages 7 and 8.
Burley allegedly stabbed the 8-year-old, who was found dead in a home elsewhere, WTAE reported. The 7-year-old reportedly fled to a neighbor's house.
The man — who was reportedly on parole for homicide — was captured by the U.S. Marshals Service in Youngstown, Ohio.