The Dark Night Rises, in the Century Theater Cinema 16 in the town of Aurora, Colorado in the early hours of Friday morning.
He also hurled a gas canister into the theater before he began shooting.
Oates said that ten people were killed at the cinema, while two others died after they were transferred to a nearby hospital.
The police arrested the 24-year-old masked gunman, who was wearing a bullet-proof vest, in the parking lot of the cinema.
According to a US Defense Department statement, three members of the US military were injured in the incident.
"The Navy and the Air Force are working with the families of these service members to ensure they have the care and attention they need," the statement said.
The man who killed 12 people and injured 59 others at a movie theater in a Denver suburb in the largest mass shooting in US history also booby-trapped his apartment, police in the state of Colorado say.
“It’s booby-trapped with various incendiary and chemical devices and trip wires,” the police chief of the town of Aurora said on Friday in describing James Eagan Holmes’ apartment, CNN reported.
Police Chief Dan Oates added that it could take many days to work through the apartment safely.
He went on to say that five buildings around Holmes’ apartment in the town of Aurora were evacuated.
Holmes was studying neuroscience in a Ph.D. program at the University of Colorado-Denver graduate school, according to university spokeswoman Jacque Montgomery.
In the incident, Holmes, who was armed with an assault rifle, a shotgun, and a pistol, opened fire during a screening of the new Batman movie,