The Colorado shooting suspect planned the rampage that killed 12 midnight movie-goers, including a six-year-old girl with "calculation and deliberation", police said.
For months James Holmes received deliveries that authorities believe armed him for battle and were used to rig his flat with dozens of bombs.
Experts were still working to clear dangerous explosive materials from inside Holmes' suburban Denver home, two days after police said he opened fire and set off gas canisters in a cinema minutes into the premiere of Batman film The Dark Knight Rises. Fifty-eight people were injured.
His flat 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 probably been one of his officers. "You think we're angry? We sure as hell are angry," Mr Oates said.
Authorities would not discuss a motive for one of the deadliest mass shootings in recent US history, as makeshift memorials for the victims sprang up and relatives began to publicly mourn their loved ones.
Holmes, 24, had recently withdrawn from a competitive graduate programme in neuroscience and neighbours and former classmates in California have said he was an intelligent loner who said little. But he had apparently prepared the attack at the Aurora multiplex cinema well in advance, receiving deliveries by post to his home and school for four months and buying thousands of rounds of ammunition on the internet, Mr Oates said.
"He had a high volume of deliveries to both his work and home address. We think this explains how he got his hands on the magazine, ammunition," he said, as well as the rigged explosives in his flat. "What we're seeing here is evidence of some calculation and deliberation," Mr Oates added.
Mr Oates said Holmes used a military-style semi-automatic rifle, a shotgun and a pistol to open fire on the unsuspecting cinema-goers. He had bought the weapons at local gun stores within the last two months and recently purchased 6,000 rounds of ammunition over the internet. Holmes also bought an urban assault vest, two magazine holders and a knife for just over 300 dollars (£192) on July 2 from an online supplier of tactical gear for police and military personnel, according to the company.
It was not known why the suspect chose a cinema to stage the assault, or whether he intended some twisted, symbolic link to the film's violent scenes.
Holmes is in solitary confinement for his protection at a county detention facility on suspicion of multiple counts of first-degree murder. He is due in court for an initial hearing on Monday and had been appointed a lawyer, authorities said.