There were 58 other people injured, according to Aurora Police Chief Dan Oates.
Oates says 10 died at the theater and two died at the area hospital.
The new Batman movie called "The Dark Knight Rises" was opening at the theater during the shooting.
9NEWS Reporter Jeremy Jojola interviewed two eyewitnesses who were in the theater when the shooting started.
"I saw a man walk in through the exit," one eyewitness told Jojola. "[He] looked like he had a gas mask on."
The witnesses say he threw a two gas canisters into the theater after busting down the door. The witnesses said no one reacted at first - thinking he was a stunt for the movie.
"He looked so calm when he did it," the eyewitness said. "It was like scary. He waited for both the bombs to explode before he did anything. Then, after both of them exploded, he began to shoot."
The witnesses said he - at first - fired into the air and then started shooting towards the people.
"He had no specific target. He just started letting loose," the eyewitness said.
Both witnesses said one of the scariest parts was the shooter didn't say a word.
"Honestly, I thought I was gonna get shot," the eyewitness said. "I thought there was no way I was going to get out of there without getting shot."
Nicole Williams, with Swedish Medical Center, says the hospital was alerted to a mass casualty incident at 1:15 a.m. Friday.
"This is ridiculous," the eyewitness told Jojola. "We really want to know what was the point of it? There's no sense to it. We have no idea what to make of it. He's shooting little kids, he's shooting adults, he's shooting our friends we went to school with. It's just sad."
Both of the eyewitnesses grew up going to this movie theater.
"It was just a terrible sight," the eyewitness said.