AURORA, Colo. (AP) — Details emerging on the Colorado massacre suspect show a budding scientist, brimming with potential, who pursued a graduate program even as he assembled weaponry he would allegedly use in the deadly midnight rampage inside an Aurora movie theater.

James Holmes 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 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.

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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 Saturday afternoon, all hazards had 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 covered 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. Saturday, 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.

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 in Holmes' home state of California 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.

Neighbors and former classmates in California said although Holmes was whip-smart, he was a loner who said little and was easily forgotten — until this week.

Mary Muscari, a criminology professor at Regis University in Denver who studies mass killings, said she was not surprised Holmes was studying neuroscience and mental disorders.

"It could be he was interested in that because he knows there's something different in him," she said.

Holmes was in solitary confinement for his protection at a Denver-area county detention facility, held without bond on suspicion of multiple counts of first-degree murder. He was set for an initial hearing on Monday and has been appointed a public defender.

Among the deceased victims was a 6-year-old girl and a man who died on his 27th birthday and a day before his wedding anniversary. Families grieved and waited at hospitals, which reported at least seven still in critical condition Saturday and others with injuries that likely are permanent.

Veronica Moser-Sullivan, 6, had gone to the movies with her mother, who was drifting in and out of consciousness in a hospital intensive care unit, bullets lodged in her throat and a gunshot wound to her abdomen.

"Nobody can tell her about it," Annie Dalton said of her niece, Ashley Moser. "She is in critical condition, but all she's asking about is her daughter."

Veronica had just started swimming lessons on Tuesday, Dalton said.

"She was excited about life as she should be. She's a 6-year-old girl," her great aunt said.

Another victim, 27-year-old Matt McQuinn, was killed after diving in front of his girlfriend and her older brother to shield them from the gunfire, said his family's attorney, Rob Scott of Dayton, Ohio.

Alex Sullivan had planned a weekend of fun, first ringing in his 27th birthday with friends at the special midnight showing of "The Dark Knight Rises" and then celebrating his first wedding anniversary on Sunday.

"He was a very, very good young man," said Sullivan's uncle, Joe Loewenguth. "He always had a smile, always made you laugh. He had a little bit of comic in him."

President Barack Obama, who called in his weekly radio address for prayer and reflection on the rampage, was scheduled to travel to Colorado on Sunday to visit with the families of victims.

During the attack early Friday, Holmes used a military-style semi-automatic rifle, a shotgun and a pistol to open fire on the unsuspecting theater-goers, Oates. Holmes had bought the weapons at local gun stores within the past two months. He recently purchased 6,000 rounds of ammunition over the Internet, the chief said.

Holmes also bought an urban assault vest, two magazine holders and a knife for just over $300 on July 2 from an online supplier of tactical gear for police and military personnel, according to the company.

Chad Weinman, CEO of TacticalGear.com, said his company processes thousands of orders each day, and there was nothing unusual in the one that Holmes placed.

"Everything Mr. Holmes purchased on July 2 is commercially available," Weinman said, adding he was "appalled" that the material was sold to Holmes before the shooting.

The Batman movie, the last in the trilogy starring Christian Bale, opened worldwide Friday with midnight showings in the U.S. "The Dark Knight Rises" earned $30.6 million in Friday morning midnight screenings and according to industry estimates roughly $75 million on that day. That put it on track for a weekend total of around $165 million, which would be the second-highest opening weekend ever, following "The Avengers."

The shooting was the worst in the U.S. 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.

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Associated Press contributors to this report include Kristen Wyatt, Steven K. Paulson, Ivan Moreno, P. Solomon Banda and Gillian Flaccus in Aurora; Dan Elliott, Nick Riccardi and Colleen Slevin in Denver; AP Entertainment Writer Jake Coyle in New York; M.L. Johnson in Chicago; Brian Skoloff in Salt Lake City; Monika Mathur and Jennifer Farrar at News Research Center in New York; and Alicia A. Caldwell and Eileen Sullivan in Washington.