Colorado massacre gunman entered theater ready to kill even more, source says

The deadly shooting attack last week at a midnight movie screening in Colorado could have been even worse if the casualties had matched the gunman's extensive firepower, one of the first emergency responders at the scene told Fox News.

James Holmes, 24, is accused of opening fire just past midnight early Friday in the Century 16 theater in Aurora, Colo., killing 12 and injuring 58. He made his first court appearance Monday in a court case that could include the death penalty.

The first responder who spoke to Fox News on condition of anonymity said the gunman began the attack by tossing tear gas canisters into the theater, then firing his 12-gauge shotgun at the ceiling before turning it on the crowd.

Other details, according to the source:

-- At some point, the gunman transitioned to his other weapons, a .40 Glock pistol and a .223 Smith & Wesson M&P semi-automatic.

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-- The shooter had purchased an after-market high-capacity drum clip for the semi-automatic, the source said. Although it gave him the capacity to fire perhaps 100 rounds, it was of inferior quality and might be the reason the shooter's gun jammed. Authorities found live .223 ammunition on the theater floor, possibly resulting from him yanking off the drum to try to clear the jam, or possibly from a "catastrophic failure" of the drum's feed mechanism.

-- The .223 rounds he was firing were armor-piercing.

-- After the gun jammed, the gunman walked out of the theater through the door he'd entered and was removing his body armor beside his car when he was confronted by the officers who took him down, the source said, adding that the gunman seemed surprised authorities arrived so quickly.

-- The gunman's apartment was booby-trapped, and authorities are considering a theory that he intended the apartment to blow first to create a diversion that would occupy police and rescue personnel several miles away from the theater, the source said.

A photo of the interior of the suspect's apartment reviewed by Fox News showed what appeared to be small, dark balls, looking like small bowling balls, placed across the floor, evenly spaced like chess pieces. Wires criss-crossed above them in what looked like a grid, with a glass beaker of liquid visible along with what appeared to be a five-gallon gas can.

Fox News has learned that the door was wired with a booby-trap and a backup system that would have triggered an explosive designed to "cut in half" the first person through the door. After that, explosions and flames would have likely consumed the entire building, presumably with the intention of trapping other residents as they slept and forcing a massive response of police and rescue personnel.