Apparently, Holmes told police during his arrest he had booby-trapped the third-floor apartment.
Kaitlyn Fonzi, a 20-year-old grad student and neighbor, said she heard techno music blasting from his apartment around the time of the shooting, and had nearly opened Holmes' unlocked apartment door to complain, unaware that the unit was booby trapped with explosives.
A timer had turned the music on so that it would blare in his apartment after he left for the Aurora multiplex, according to a law enforcement source, who also spoke on condition of anonymity.
Bomb technicians worked Saturday to clear "all hazards" from the apartment, and began allowing some evacuated residents to return.
Jackie Mitchell, who lives close to Holmes, had a beer with him on Tuesday.
Mitchell was stunned at news of Holmes' alleged involvement in the attack.
"You would never guess he was a violent guy," Mitchell said, describing Holmes as "nerdish" and "a book-smart type guy."
In San Diego, the suspect's family issued a statement saying they were still trying to process the news.
"Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved," the Holmes family said, without giving any information about him.