Four people are dead and two are hospitalized in stable condition following a mass shooting late Monday night in the Kingsessing neighborhood of Philadelphia, officials said.

Philadelphia Police Commissioner Danielle M. Outlaw said two people were in custody: a 40-year-old man who is believed to be the shooter, and a second person who authorities believe picked up a firearm and started shooting at the suspect. Police said the suspect was wearing a bulletproof vest and had multiple magazines, and was firing randomly.

"Officers responded quickly and showed bravery and courage," Outlaw said during a press conference, adding, "Their actions prevented potentially more victims."

Police received reports of gunfire in the area behind Chester Street around 8:56 p.m. ET. Officers responded to the scene and found the gunshot victims, who have not yet been identified.

Of the four deceased victims, three are between 20 and 59 years old, while the fourth victim may be between 16 and 21, authorities said. All are male. The two stable victims, who are also male, are children ages 2 and 13.

This is a developing story. Please check back for updates.