The shooter, a man in his 20s, was arrested shortly after the midnight attack near a car outside the multiplex theatre in Aurora, suburban Denver.
Speaking at the scene where police were working to gain access to the apartment, Aurora Police Chief Daniel Oates said: "His apartment is apparently booby trapped. And what we have is a whole bunch of bomb techs from all different agencies, we have pictures inside the location, and we are trying to determine how to disarm the flammable or explosive material that is in there.
"We could be here for hours, we could be here for days, trying to figure out how to get in there. The the pictures are pretty disturbing, it looks pretty sophisticated on how it's booby-trapped.
"We evacuated, we consulted the experts, we evacuated five buildings total and we believe that is a sufficient evacuation for what we see inside. But beyond that, this could be a very long wait. We don't know how long we are going to be here."