Neighbors and acquaintances have said that Holmes was quiet, lived alone and kept to himself. No close friends have yet come forward to talk about him.
Holmes Began Amassing an Arsenal Two Months Ago
Holmes bought his first gun, a glock pistol, at Gander Mountain Guns in Aurora two months ago.
Over the next several weeks, he bought more guns at Colorado gun shops -- a tactical shotgun, another pistol and the suspected primary murder weapon -- a Smith and Wesson high powered assault rifle.
Holmes does not have a criminal record except for a traffic violation, which would allow him to pass any background check for weapons, law enforcement sources said.
Sources tell ABC News that Holmes also bought 6,000 rounds of ammunition and swat gear, including body armor and a gas mask, through the internet.
When he was arrested Friday, he indicated to police that he had booby-trapped his apartment, leading officers to evacuate the Aurora apartment building.
Authorities found that the apartment was laced with intricate and potentially deadly booby-traps.
"I see an awful lot of wires, trip wires, jars full of ammunition, jars full of liquid. Some things that look like mortar rounds," Aurora Police Chief Dan Oates said. "We have a lot of challenge, to get in there safely."
Holmes is being held in jail and is scheduled to make his first court appearance on Monday.