'The whole building would have burned'

After the shooting rampage, police went to Holmes' apartment, where they found a series of booby traps that prevented them from entering for two days.

Sources familiar with the investigation said that some 30 shells filled with gunpowder were spread through the 800-square-foot apartment and wired to a control box in the kitchen.

There were also at least two containers filled with "incendiary liquids" meant to fuel a fire from the initial explosions, and an undetermined amount of bullets meant to ricochet around the apartment.

An FBI agent takes photographs inside the apartment of James Holmes, the suspect who opened fire in a movie theater in Aurora, Colorado, July 21, 2012. [Photo/Agencies]

"Given the amount of explosives that were there, if they detonated ideally, you would have had a very ample explosion with an ensuing thermal effect from the incendiary liquids that would have destroyed that apartment complex," a law enforcement official said.

"It's safe to say that that whole building would have burned," the official said. "The explosion would have certainly removed the interior walls in the building and some of the exterior walls."

Local and federal bomb experts used a robot to disable a trip wire, then had the robot place a tube - known as a "water shot" - near the control box, authorities said. The water shot was then detonated to disable the control box.

Police evacuated five buildings nearby and created a perimeter of several blocks around Holmes' apartment, the top-floor unit of a three-storey red brick building in a run-down section of Aurora.

"This apartment was designed to kill whoever entered it," Oates said.