According to various reports, at approximately 12:30 a.m. a man in a gas mask stood in front of a crowd during a screening of the film at a mall in Aurora and went on a shooting rampage. He allegedly set off a smoke bomb before opening fire.
Ten were reportedly killed at the scene, and the rest died at local hospitals, including a 6-year-old.
A man has been arrested in a parking lot behind the multiplex, police spokesman Frank Fania told reporters.
The suspect, who police did not immediately identify but is believed to be in his early 20s, was reportedly wearing a bulletproof vest and carrying a rifle, handgun and knife. Another gun was purportedly recovered at the theater.
"He did not resist. He did not put up a fight," Fania said.
Authorities also confirm that there was no evidence that a second gunman was involved.
Eyewitnesses told CNN that the gunman reportedly kicked open an exit door and flung a smoke bomb into the theater before making his way through the crowd and randomly opening fire.
"We saw people running around and screaming," one eyewitness said.
Another recalls hearing "anywhere from 10 to 20 shots and little explosions going on. Shortly after that we heard people screaming. Then they came on PA system and said everyone needed to get out."
A spokeswoman for a local hospital confirmed that all of the victims suffered gunshot wounds ranging from minor to critical.
The FBI has now joined in the investigation.
In a statement, Warner Bros., the studio behind the movie, 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."