A 22-year-old woman was fatally shot Thursday night while answering the door of her home in West Philadelphia, police said.

The woman was responding to a knock about 7 p.m. in the 5800 block of Walton Avenue when she was shot once in the chest by an unidentified male, said Chief Inspector Scott Small.

Her 4-year-old son was in the house but was not hurt, Small said.

Officers drove the woman to Penn Presbyterian Medical Center, where she was pronounced dead at 7:37 p.m.

Two adult relatives in the house said they did not hear any argument before the shooting, Small said.

Police found three shell casings next to the door, Small said. Police were checking nearby surveillance cameras.