Chicago police have released from custody a man suspected of shooting three passengers on a city bus after determining he wasn't the gunman.

Officers had taken the 35-year-old man into custody after witnesses said he was possibly the gunman who fired several shots at the bus just after getting off on Monday on the city's South Side. But police spokesman Anthony Spicuzza says the bus surveillance video showed he wasn't the shooter.

An 18-year-old man was taken to the University of Chicago Medical Center in critical condition, but his condition stabilized. An 18-year-old woman was treated for a wound to the underarm, and a bullet grazed a 59-year-old man.

Police initially said the shooting stemmed from a quarrel but later said they didn't know the motive.