By THOMAS PEIPERT and MEAD GRUVER
AURORA, Colo. (AP) - The shooting suspect accused in a deadly rampage inside a Colorado theater planned the attack with "calculation and deliberation," police said Saturday, receiving deliveries by mail that authorities believe armed him for battle and were used to rig his apartment with dozens of bombs.
Meanwhile, a federal law enforcement official provided an updated account about the gunfire inside the theater, saying that a semi-automatic assault rifle used by the shooter jammed during the attack, forcing him to switch to another weapon.
The official, who spoke on condition of anonymity to in order to discuss the investigation, said late Saturday night that the jammed 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 weapon would be able to fire 50 to 60 rounds a minute.
In Aurora, investigators spent hours Saturday removing explosive materials from inside Holmes' suburban Denver apartment a day after police said he opened fire and set off gas canisters in a theater minutes into a premiere of the Batman film "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, noting it would have likely been one of his officers.
Holmes received several mail deliveries over four months to his home and school and bought thousands rounds of ammunition on the Internet.
"He had a high volume of deliveries," Oates said. "We think this explains how he got his hands on the magazine, ammunition," he said, as well as the rigged explosives in his apartment.
"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" and an improvised explosive device containing an unknown substance. There also were multiple containers of accelerants.
"It was an extremely dangerous environment," Yacone said at a news conference, noting that anyone who walked in would have sustained "significant injuries" or been killed.
By late afternoon, all hazards have been removed from the Holmes' apartment and residents in surrounding buildings were allowed to return home, police said.
The exception was Holmes' apartment building, where authorities were still collecting evidence. Inside the apartment, authorities began covering the windows with black plastic to prevent onlookers from seeing in. Before they did, a man in an ATF T-shirt could be seen measuring a poster on a closet that advertised a DVD called "Soldiers of Misfortune." The poster showed several figures in various positions playing paintball, some wearing masks.
About 8 p.m., police left the apartment building carrying a laptop computer and a hard drive.
While authorities continued to refuse to discuss a possible motive for one of the deadliest mass shootings in recent U.S. history, details about Holmes' background as a student and would-be scientist trickled out Saturday.
Holmes 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. He recently took an intense three-part, oral exam that marks the end of the first year of the four-year program there, but university officials would not say if he passed, citing privacy concerns. The university said Holmes gave no reason for his withdrawal, a decision he made in June.
"The focus of the program is on training outstanding neuroscientists and academicians who will make significant contributions to neurobiology," the university said. The doctoral program usually takes five to seven years to complete, it said.
In a resume posted on Monster.com, Holmes listed himself as an "aspiring scientist" and said he was looking for a job as a laboratory technician.
The resume, first obtained by The Press-Enterprise in Riverside, 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 an undergraduate at the University of California, Riverside.
He also worked as a cabin counselor to underprivileged children at a summer camp in Los Angeles in 2008. In a statement, Camp Max Straus confirmed Holmes had worked there for eight weeks. The camp provided no other detail about Holmes but said such counselors are generally responsible for the care and guidance of roughly 10 children.