The semiautomatic assault rifle used by the gunman in a mass shooting at a midnight showing of the latest Batman movie jammed during the attack, forcing the shooter to switch to another gun with less fire power.
A US federal law enforcement official said the disabled weapon had a high-capacity ammunition magazine. Police have said that a 100-round drum magazine was recovered at the scene and that such a device would be able to fire 50 to 60 rounds a minute.
That account of what happened inside the Century 16 theater emerged with other details of a suspect described as a budding scientist, brimming with potential, who pursued a graduate program even as he planned the attack with "calculation and deliberation," police said.
James Holmes, 24, received shipments that authorities believe armed him for battle and were used to booby trap his home.
In Aurora, investigators spent hours on Saturday removing explosive materials from inside Holmes' apartment a day after police said he opened fire and set off gas canisters minutes into a premiere of "The Dark Knight Rises." The massacre left 12 people dead and 58 injured.
His apartment was rigged with jars of liquids, explosives and chemicals that were booby trapped to kill "whoever entered it," Aurora Police Chief Dan Oates said.
Holmes received several mail deliveries over four months to his home and school and bought thousands of rounds of ammunition on the Internet.
"He had a high volume of deliveries," Oates said.
"What we're seeing here is evidence of some calculation and deliberation," Oates added.
Inside the apartment, FBI Special Agent James Yacone said bomb technicians neutralized what he called a "hypergolic mixture" - substances that would spontaneously ignite on contact - and an improvised explosive device containing an unknown substance. There were also multiple containers of accelerants. "It was an extremely dangerous environment," Yacone told reporters.
By late Saturday afternoon, all hazards had been removed from Holmes' apartment and residents in surrounding buildings had been allowed home, police said.
The exception was Holmes' building, where authorities were still collecting evidence. Authorities covered the windows with black plastic to prevent onlookers seeing in. Before they did, a man in an ATF T-shirt could be seen measuring a poster for a DVD called "Soldiers of Misfortune." The poster showed several figures in various positions playing paintball, some wearing masks.
While authorities continued to refuse to discuss a possible motive for one of the deadliest mass shootings in recent US history, details about Holmes' background as a student trickled out.
He had recently withdrawn from a competitive graduate program in neuroscience at the University of Colorado, Denver, where he was one of six students at the school to get National Institutes of Health grant money.
In a resume posted on Monster.com, Holmes listed himself as an "aspiring scientist" and said he was seeking a job as a laboratory technician.
The resume paints a picture of a brilliant young man brimming with potential. He worked as a summer intern at the Salk Institute for Biological Studies in La Jolla in 2006 and mapped the neurons of zebra finches and studied the flight muscles of hummingbirds while at the University of California, Riverside.
He also worked as a cabin counselor to underprivileged children in Los Angeles in 2008.
Holmes is in solitary confinement at a Denver-area county detention facility, held on suspicion of multiple counts of first-degree murder. An initial hearing is set for today.