Another estimated 50 were wounded, including a 6-year-old girl, Police Chief Dan Oates told reporters, CNN reports.
Police apprehended 24-year-old suspect James Holmes – a local resident who was said to be heavily armed and wearing a bulletproof vest – in a rear parking lot of the Century 16 Theater in Aurora's Town Center Mall, police spokesman Frank Fania told CNN. "He did not resist. He did not put up a fight," Fania said.
Holmes remains in custody. His home is currently being searched for possible explosive materials or devices.
The attack in the theater began when the suspect tossed a canister of tear gas into the crowd through an emergency exit door, reports CNN affiliate KUSA. He then opened fire. Some moviegoers thought he was part of the show.
"We saw people running around and screaming," an unidentified man told KUSA. Another moviegoer said the gunman wore a gas mask and moved through the theater, firing randomly.
Of the wounded, at least 20 were being treated at the University of Colorado Hospital for minor to critical gunshot wounds, spokeswoman Jacque Montgomery told CNN.
At the moment, the Dark Knight Rises cast – including Christian Bale, Anne Hathaway, Joseph Gordon-Levitt, Morgan Freeman and Marion Cotillard – is in Paris for a scheduled red carpet premiere on the Champs-Elysees Friday night. PEOPLE has been told that Warner Bros. has canceled all interviews related to the movie opening, as well as the premiere itself.
In a statement, the studio said: "Warner Bros. is deeply saddened to learn about this shocking incident. We extend our sincere sympathies to the families and loved ones of the victims at this tragic time."
Additional reporting by PETER MIKELBANK