COLORADO, USA (AP) — The shooting suspect accused in a deadly rampage inside a Colorado theatre 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, new details about 24-year-old James Holmes emerged, including summer jobs the suspect held in Southern California as a camp counsellor and as an intern at a prominent research institute.

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 theatre minutes into a premiere of the Batman film "The Dark Knight Rises”. The attack 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 neutralised what he called a "hyperbolic 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 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 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.

Holmes was in solitary confinement for his protection at a county detention facility Saturday, 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.

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