Holmes was studying neuroscience in a Ph.D. program at the University of Colorado-Denver, university spokeswoman Jacque Montgomery said. Holmes enrolled a year ago and was in the process of withdrawing at the time of the shootings, Montgomery said.
Authorities gave no motive for the attack. The FBI said there was no indication of ties to any terrorist groups.
Holmes had an assault rifle, a shotgun and two pistols, a federal law enforcement official said, speaking on the condition of anonymity because the investigation was still unfolding.
FBI agents and police used a hook and ladder fire truck to reach Holmes' apartment in Aurora, police Chief Dan Oates said. They put a camera at the end of a 12-foot pole inside the apartment and discovered the unit was booby-trapped. Authorities evacuated five buildings as they tried to figure how to disarm the flammable and explosive material.
"[The suspect] lives on the third floor in the back," Oates said. "We have a whole bunch of bomb techs from a whole bunch of agencies. WE have pictures inside the location. We are trying to determine how to disarm flammable or explosive material that's in there. We could be here for hours. We could be here for days trying to get in there and get whatever evidence there is. The pictures are pretty disturbing. It looks pretty sophisticated in terms of how it's booby-trapped."
Police evacuated the building and surrounding buildings as a precaution.
"This could be a very long wait," Oates said. "We don't know how long we are going to be here."
Deputy Fire Marshal Chris Hendersen says they are searching all the floors of the apartment.
"We already searched but are doing it one more time to make sure there are no more occupants in the building," Hendersen said. "Once that's cleared, we've got the bomb squad who will send in a robot. The actual apartment that we are concentrating on is pretty extensively booby-trapped."
Hendersen says there's a chance that robot could detonate the explosive materials.
"[There are] several bottles of an unknown liquid spaced in different places across the floor and also some other potentially explosive devices, and some kind of wire connected to those," Hendersen said.
Hendersen says they still do not know how extensive the booby-traps are.
Neighbors told 9NEWS they heard very loud techno music coming from Holmes' apartment unit. They say the music ended abruptly around 1 a.m.
Neighbors speculate the music was intended to lure apartment residents to Holmes' door, where a possible explosive or booby trap would have been detonated.
Police on Friday evening escorted residents seeking to gather personal items into several apartment buildings in the area that were evacuated.
(KUSA-TV © 2012 Multimedia Holdings Corporation with The Associated Press)