Officers discovered that his Aurora home was booby-trapped after using a camera on the end of a 12-foot pole to complete an initial inspection.
© PA Images / Ed Andrieski/AP
© PA Images / Barry Gutierrez/AP
Technicians attempted to disarm the traps - believed to include explosives - on Friday but halted their efforts as it was too dangerous.
Federal government experts will arrive at the scene today (July 21) for the second attempt, the BBC reports.
Aurora police chief Dan Oates said that the apartment contained "things that look like mortar rounds", adding: "I've personally never seen anything like what we've found in there."
Other residents from the building, which is about five miles from the cinema, have been evacuated along with four nearby premises. Meanwhile, firefighters are monitoring the apartment block for gases to identify any chemicals used in the booby-traps.
Holmes remains in police custody after a man wearing a gas mask and body armour threw tear gas canisters in the cinema and then began shooting at the audience. He will appear in court on Monday.
The 24-year-old had bought 6,000 rounds of ammunition over the internet, Oates added. All his weapons and ammunition had been purchased legally within the past few months.
Thirty people remain in hospital following the attack - 11 are in a critical condition.
