The US Federal Bureau of Investigation is collecting evidence from the home of the man believed to have shot dead 12 people in a cinema in the town of Aurora in Colorado.
The suspect, James Holmes, is due before a court tomorrow.
It took police two days of delicate work to clear a sophisticated booby-trap bomb at his apartment.
US President Barack Obama travelled to Colorado today to meet families bereaved in the shooting.
The victims ranged in age from six to 51-years-old. They were shot by a gunman who opened fire on film-goers during a packed premiere of the latest Batman film on Friday.
A vigil was scheduled for 6.30pm today (1.30am tomorrow Irish time) in front of Aurora city hall, organised by civic, community, and religious leaders.
"We can now start the natural process of grieving and healing," Aurora Mayor Steve Hogan said at a memorial late last night for one young shooting victim. "We're still reeling."
Graduate school dropout James Holmes, 24, was arrested immediately after the fatal attack at the local multiplex.
Aurora Police Chief Dan Oates said the shooting followed months of "calculation and deliberation," as Mr Holmes received a "high volume" of deliveries to both his work and home.
Residents of several nearby buildings were allowed to return home yesterday, while the red-brick apartment block where Mr Holmes lived remained under evacuation as local and federal authorities completed the process of disarming the explosives and sifting through evidence.
Sources familiar with the probe said that 30 shells filled with gunpowder were found in the apartment, together with containers filled with "incendiary liquids" intended to fuel a fire from the initial explosions, as well as bullets meant to ricochet around the apartment.
Yesterday afternoon the local coroner's office released the names of the 12 people killed, including those of a six-year-old girl, a young man celebrating his 27th birthday and an aspiring sports-caster who had escaped a shooting in a Toronto mall earlier this summer.
The mass shooting stunned the nation.
It also reverberated in the US presidential race. Both President Obama and his Republican rival, Mitt Romney, toned down their campaigns on Friday, pulled ads from Colorado and dedicated scheduled events to the victims.
Those who witnessed the shooting told of a horrific scene, with dazed victims bleeding from bullet wounds, spitting up blood and crying for help.
Mr Holmes was arrested minutes later in a car park behind the cinema.
He was armed with an AR-15 assault rifle, a 12-gauge shotgun and a Glock .40-caliber handgun, Police Chief Oates said.
Police found an additional Glock .40-caliber handgun in his car. All the weapons were legally bought in the past 60 days.
Arapahoe County Sheriff Grayson Robinson said the suspect was being held in solitary confinement to protect him from other prisoners, a routine move in high-profile cases.
Mr Holmes, who authorities said had dyed his hair red and called himself "the Joker" in a reference to Batman's comic-book nemesis, was due to make an initial court appearance tomorrow.
Little has surfaced from his past to suggest he was capable of such violence.
Until last month, he was studying for a doctoral degree in neuroscience at the University of Colorado's Anschutz Medical School, a few blocks from his apartment.
The University of Colorado Hospital, which treated some of the shooting victims, said ten people had been released and five remained in critical condition.
The Medical Center of Aurora said four of its seven patients remained in the intensive care unit, while three others were on the main trauma floor.
A memorial of flowers, candles and stuffed animals quickly sprung up where the shooting rampage took place. A handwritten sign read: "7/20 gone not forgotten."