Holmes' 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.
"You think we're angry? We sure as hell are angry," Oates said.
Authorities wouldn't discuss a motive for one of the deadliest mass shootings in recent U.S. history, as makeshift memorials for the victims sprang up and relatives began to publicly mourn their loved ones. Holmes had recently withdrawn from a competitive graduate program in neuroscience; neighbors and former classmates have said he was a smart loner.
He apparently had prepared the attack well in advance, receiving multiple deliveries by mail for four months to his home and school and buying thousands of rounds of ammunition on the Internet, Oates said.
"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 said.
Federal authorities detonated one small explosive and disarmed others inside Holmes' apartment after sending in a robot to take down a trip wire, FBI Special Agent James Yacone said. Bomb technicians then neutralized what he called a "hyperbolic mixture" and an improvised explosive device containing an unknown substance. There also were multiple containers of accelerants, he said.
"It was an extremely dangerous environment," Yacone said, adding that anyone who walked in would have suffered "significant injuries" or been killed.
Outside the apartment, police arranged plastic storage boxes and large white plastic bags, possibly for evidence.
Police said in a statement Saturday evening that "major threats have been eliminated" from the apartment, but the overall threat has not been completely eliminated.
Holmes, 24, 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 had been appointed a public defender, authorities said.
Oates said Holmes used a military-style semiautomatic rifle, a shotgun, and a pistol to open fire on the unsuspecting theatergoers. He had bought the weapons at local gun stores within the last 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. While his company often receives orders from military units and law-enforcement organizations, it is not out of the ordinary for individual police officers or soldiers to place orders, he said.
"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.
It wasn't known why the suspect chose a movie theater to stage the assault, or whether he intended some twisted, symbolic link to the film's violent scenes.
The Batman movie, the last in the trilogy starring Christian Bale, opened worldwide Friday with midnight showings in the United States.
The Dark Knight Rises earned $30.6 million in Friday morning midnight screenings, and, according to industry estimates, roughly $75 million to $77 million on the 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.
Warner Bros. has announced it would forgo the usual revenue reports until Monday out of respect for the victims. Sony, Disney and Universal also said they would delay reporting box office receipts until Monday, a day later than the routine Sunday releases for Hollywood.