A gunman opened fire at a first-night screening of the The Dark Knight Rises in Aurora, Denver, leaving 12 people dead and many more injured.
The shootings took place at the Century Theater at the Aurora Mall in Denver around 7am British time where moviegoers had arrived to see a midnight showing of the much-anticipated new Batman film.
Aurora Police Chief Dan Oates had confirmed 14 fatalities and at least 45 injured cinemagoers, but the Colorado Police Department have recently revised the death toll down to 12, saying that a total of 50 people had been shot.
Witnesses said a man wearing a gas mask began firing at members of the audience during the film, with some reports suggesting there was smoke or gas released. KUSA, an NBC-affiliated local news channel, has since reported that some hospitalised victims were being treated for chemical exposure, believed to be tear gas.
A man, named by the FBI as James Holmes, 24, has since been arrested. Frank Fania, a police spokesperson, said: "He was in the rear parking lot which really is not a public parking lot for the theatre. He did not resist, he did not put up a fight."
Fania also added that a rifle, a handgun and another gun had been found in the cinema.
The gunman's motive remains unknown, but a spokesman for the FBI, Jason Pack, said there was no indication so far of any connection to terrorism.
The Century Theater is located within 20 miles of Columbine High School where two students shot dead 13 people in 1999.
President Obama has released a statement urging citizens to "come together as one American family".