A 6-year-old boy identified only as Domenic died in the predawn incident and his 12-year-old sister was in critical condition.
WPVI-TV in Philadelphia said a manhunt was launched for the suspect, whom neighbors identified as a young man nicknamed Papi who lives in the neighborhood and was never considered a threat. "People would never think he would do something like this," said neighbor Nakyta McCray. "The kids are always around him and everyone knows him."
Witnesses told WPVI the injured girl, who turned 12 and is named Amber, managed to identity Papi as her assailant.
The children had been home with a teenage sister while their mother recovered in a hospital from surgery. The 14-year-old girl was not injured.