"He looked like an assassin ready to go to war," said Jordan Crofter, a moviegoer who was unhurt in the attack early Friday, about a half-hour after the special midnight opening of The Dark Knight Rises.
The suspect, identified by police as James Holmes, 24, used a military-style semiautomatic rifle, a shotgun, and a pistol, stopping only to reload.
The suspect marched up the aisle in the stadium-style theater, picking off those who tried to flee, witnesses said. Authorities said he hit 71 people. One was struck in an adjacent theater by gunfire that went through the wall.
"He would reload and shoot, and anyone who would try to leave would just get killed," said Jennifer Seeger, adding that bullet casings landed on her head and burned her forehead.
Within minutes, frantic 911 calls brought about 200 police officers, ambulances, and emergency crews to the theater. Holmes was captured in the parking lot. Police said they later found that his apartment was booby-trapped.
Authorities gave no motive for the attack. The FBI said there was no indication of ties to any terrorist groups.
In New York, Police Commissioner Raymond Kelly said: "It clearly looks like a deranged individual. He has his hair painted red. He said he was the Joker, obviously the enemy of Batman."
Aurora Police Chief Dan Oates would not confirm that information, but did say he had spoken to Kelly. The two used to work together in New York. Asked whether Holmes had makeup to look like the Joker, Oates said: "That to my knowledge is not true."
It was the worst mass shooting in the United States since the Nov. 5, 2009, attack at Fort Hood, Texas. An Army psychiatrist was charged with killing 13 soldiers and civilians and wounding more than two dozen others.
It was the deadliest in Colorado since the Columbine High School massacre in suburban Denver in 1999, when two students killed 12 classmates and a teacher and wounded 26 others before killing themselves.
The new Batman movie, the last in the trilogy starring Christian Bale, opened worldwide Friday with midnight showings in the United States. The plot has the villain Bane facing Bale's Caped Crusader with a nuclear weapon that could destroy all of fictional Gotham.
The shooting prompted officials to cancel the red-carpet premiere in Paris, and some U.S. theaters stepped up security for daytime showings.
The attack began shortly after midnight at the multiplex in Aurora, an urban community on Denver's east side. Audience members said they thought it was part of the movie, or some kind of stunt associated with it.
The film has several scenes of public mayhem - a hallmark of superhero movies. In one scene, Bane leads an attack on a stock exchange, and in another he leads a shooting and bombing rampage on a packed football stadium.
A federal law enforcement official said Holmes bought a ticket to the show, went into the theater as part of the crowd, and propped open an exit door as the movie was playing. The suspect then donned protective ballistic gear and opened fire, the official said, speaking on condition of anonymity to discuss the ongoing investigation.
At some point, the gunman appeared to have stepped outside, because several witnesses saw him come through the door.
"All I saw is the door swinging open and the streetlights behind, and you could see a silhouette," said Crofter, who was sitting on the left side of the theater and toward the front.
Sylvana Guillen said the gunman, clad in dark clothing, appeared at the front of the theater as the character Catwoman appeared in the movie. Then they heard gunshots and smelled smoke from a canister he was carrying.
As she and her friend, Misha Mostashiry, ran to the exit, Guillen said, they saw a man slip in the blood of a wounded woman he was trying to help.
Oates said the gunman wore a gas mask and a ballistic helmet and vest, as well as leg, groin, and throat protectors. He said that among the guns was an assault rifle and that the gunman used two gas canisters.
Authorities started to remove the bodies from the theater on Friday afternoon. Officials wheeled a black bag on a stretcher out of the front entrance, placing it in the back of a minivan. Ten people died in the theater, while two others died later from their injuries.
Holmes lived in an apartment in Aurora, and FBI agents and police who went there discovered it was booby-trapped when they used a camera at the end of a 12-foot pole to look inside.