After clearing "all hazards" from the booby-trapped apartment of a gunman who went on a shooting spree at a US cinema hall, police entered the "designed to kill" house where the suspect had planned the deadly attack with "calculation and deliberation" for months.
Two days after 12 people, including a six-year-old girl, were killed and 58 others wounded during the premiere of new Batman movie The Dark Knight Rises at an Aurora theatre in Denver, Colorado, US President Barack Obama was set to visit the injured and families who lost their loved ones.
On Saturday night, investigators carried out "controlled detonation" in Holmes' booby-trapped one-bedroom apartment after which they entered the house in a bid to find out clues to the motive behind the deadly attack.
At least 30 improvised explosive devices were found when police entered the Denver apartment of Holmes, who was arrested after the shooting.
Police said Holmes may have been planning the attack for a long time as he received a high volume of deliveries over the past four months to both his home and work addresses.
"What we're seeing here is evidence of, I think, some calculation and deliberation," Aurora Police Chief Daniel Oates said. "We have the evidence of a deliberative process to commit this assault, and we have the evidence of a deliberative process in his mind to attack whoever opened the door of his apartment."
"Make no mistake," Oates said, "this apartment was designed to kill whoever entered it."
Police had evacuated hundreds of residents from five apartment buildings, including the one where Holmes lived, after he told them he had rigged his apartment with explosives.
Technicians, with the help of a robot, worked to handle traps, wires and possible explosive and incendiary devices, Jim Yacone, a special agent with the FBI, told reporters. Two devices were intentionally detonated at the site.
The mission was carried out with an objective of preserving evidence, all of which will be sent to an FBI lab in Quantico, Virginia, Yacone said.
Holmes' apartment was rigged with jars of liquids, explosives and chemicals. By last night, police said that "all hazards" had been removed from the suspect's apartment.
Police also said that no other person helped Holmes prepare the massacre.
"There are multiple unconfirmed and inaccurate news reports about a second suspect," police spokeswoman Cassidee Carson said in a statement. "In regards to those reports:
An associate of Holmes was interviewed evening. There is no reason to believe that he is involved."
Describing the shooting as an "evil" act which was "beyond reason," Obama on Saturday said his government "stands ready to do everything necessary to bring whoever's responsible for this heinous crime to justice."
A vigil was organised to mourn the death of innocent victims as hundreds held candles, many sobbing and hugging each other in an outpouring of grief.
Ten people were killed inside the theatre while two others died at hospitals. Thirty people remained in hospital, Colorado Governor John Hickenlooper said. Eleven of them were in critical condition.
Christian Bale, who along with Anne Hathaway starred in The Dark Knight Rises, expressed his grief at the shooting.
"Words cannot express the horror that I feel. I cannot begin to truly understand the pain and grief of the victims and their loved ones, but my heart goes out to them," Bale said in a statement.
Authorities also released the names of those who died, including a six-year-old girl whose mother was among the injured and two US servicemen.