COLORADO: 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.

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.