Capt. John Gallagher says the 18-year-old was in an online chat early Wednesday when he shot himself with a gun he apparently believed was not loaded.
The Philadelphia Daily News reports the teen remained hospitalized in extremely critical condition.
Investigators say they're still trying to find out who the teen was chatting with when he was wounded. They want to find out if the exchange was recorded.
Gallagher says the teen's two brothers were asleep in the room at the time. Police haven't released his name.