AURORA, COLO. — 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, new details about 24-year-old James Holmes emerged, including summer jobs the suspect held in Southern California as a camp counselor 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 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 “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 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 freshman year of the four-year program there, but university officials would not say if he passed, citing privacy concerns.

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.

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.