Sgt. Michael Woody tells The Associated Press the girl was playing and watching TV on Thursday afternoon with the 4-year-old boy and his 5-year-old sister when she found the gun. Woody says she picked it up, pointed it at the boy and shot him once in the chest.
Woody says the boy was taken to a hospital and was pronounced dead.
Woody says a man who lives at the home was there at the time and is believed to a grandfather. It wasn't immediately known whether it was his gun.
Police are investigating. A review by prosecutors is expected.