24-year-old James Eagan Holmes fired indiscriminately at the moviegoers yesterday during the showing of the new Batman flick starring Christian Bale and Anne Hathaway in Aurora, a suburb of Denver, Colorado.
The man, who was described as a "loner", was a doctoral student at the University of Colorado School of Medicine in Aurora, police said.
He was associated with the school's neuroscience programme and was in the process of withdrawing at the time of the shooting, said officials of the university, where Holmes had enrolled in June 2011.
The gunman, who was dressed head-to-toe in black tactical gear, had coloured his hair red and told police he was "the Joker" after he was arrested in the rear parking lot of the theatre minutes after the shooting, according to a federal law enforcement source with knowledge of the investigation.
The Joker has long been a fixture in Batman comics and was famously brought to life by the late Heath Ledger in 2008's "The Dark Knight," the prequel to "The Dark Knight Rises."