The suspected gunman in the massacre at an Aurora, Colo., movie theater appears to have acted alone, the police chief said Sunday, adding that a computer had been recovered from his apartment, which might provide additional insight into the attack.
“All evidence we have, every single indicator is that it was all Mr. [James] Holmes’ activity and that he wasn’t particularly aided by anyone else,” Chief Dan Oates said Sunday on CBS' "Face the Nation."
As for the computer, he said: “We’re hopeful that will yield some information,” he said.
PHOTOS: 'Dark Knight Rises' shooting
Also Sunday, the Associated Press reported that the semiautomatic assault rifle used during the attack jammed during the early Friday rampage, forcing the shooter to switch to another, less powerful weapon.
Twelve people were killed in the attack during a midnight showing of the "The Dark Knight Rises," and 58 were injured.
The AR-15 assault-style rifle used in the attack was equipped with a high-capacity ammunition magazine, capable of firing 50 to 60 rounds a minute, the AP reported. But the shooter also had a shotgun and two Glock pistols. He turned to those weapons when the assault rifle malfunctioned.
WHO THEY WERE: Aurora movie theater victims
Holmes, 24, was arrested without incident in a nearby parking lot after the shooting. He was booked on suspicion of first-degree murder and is scheduled to appear in Arapahoe County Court on Monday, Arapahoe County Undersheriff David Walcher has said.
Walcher has declined to comment on how Holmes is being housed or his demeanor since he has been in custody.
Police said Holmes, carrying the AR-15 and the other weapons, walked into the theater about 20 minutes into the movie. He wore a gas mask and a ballistics helmet and vest, as well as groin, throat and leg protectors, they said. He allegedly released two smoke- or gas-emitting devices, and then opened fire, shooting at anyone who tried to escape.
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The suspect had legally purchased four weapons in the last 60 days from local gun shops, Oates has said. Holmes also had also purchased more than 6,000 rounds of ammunition -- 3,000 for the assault rifle, 3,000 for the two handguns and 300 rounds for the shotgun, officials have said.
“All weapons he possessed, he possessed legally. All clips he possessed, he possessed legally. All ammunition he possessed, he possessed legally,” Oates said Friday at a news conference.
Holmes bought several magazines on the Internet for the assault rifle, Oates said, including one 100-round drum magazine that was recovered from the scene. With the drum magazine, the gunman could have shot 50 to 60 rounds within one minute.
Holmes had received a large number of packages at home and at school in recent months, suggesting that he had spent much time and effort to plan the attack, Oates said.
Holmes, who had withdrawn from the University of Colorado Denver’s graduate program in neurosciences, left his apartment booby-trapped with explosives, police said.
Oates said investigators believe the deliveries included the ammunition used in the attack and the materials used to booby-trap the apartment.
Bomb experts worked throughout Saturday to disable the devices, including at least one trip wire and explosive or incendiary devices apparently designed to kill anyone who entered the Paris Street apartment where Holmes lived. Two small explosions could be heard while the delicate operation unfolded, in which care was taken to preserve potential evidence.
Oates said the apartment was rigged to kill and that most likely the first ones to enter would have been police officers.
"Make no mistake, this apartment was designed … to kill whoever entered it," Oates said Saturday, describing the booby traps Holmes apparently left behind.
Whoever entered the apartment "was going to be a police officer ... and if you think we're angry, we sure as hell are angry [over] what has happened to this city, what has happened to these people here and what he intended to do to our police officers."
In the "Face the Nation" interview, Oates revealed new details of the arrest outside the theater. Without elaborating, he said that there was one aspect of Holmes’ equipment that distinguished itself from what a SWAT officer responding to the scene would wear.
That distinction is what attracted attention from the officers, Oates said.
“In that chaos, it is quite reasonable that an officer might have confused him for a SWAT officer, a heavily armed officer who was responding to the scene,” he said. “So, kudos to my two cops that grabbed this guy and that sharp observation that they made immediately that led them to suspect him as being the suspect.”
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