Colorado police are still grappling with possible motives as to why James Eagan Holmes, a middle-class 24-year-old neuroscience student, may have killed 12 cinema-goers watching a Batman film.

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Holmes, with only a speeding ticket on his record, is suspected of having dressed in a gas mask, ballistic helmet, tactical ballistic vest, throat guard, leggings and crotch guard before opening fire in a suburban cinema in Aurora. He bought four guns and more than 6,000 rounds of ammunition over two months, Colorado police said.

"He had his hair painted red. He said he was the 'Joker', obviously the enemy of Batman," New York Police Commissioner Ray Kelly told reporters, referring to a character in the Batman comics who commits acts of random, chaotic violence.

The California years

Holmes grew up in San Diego, California. Neighbours told the Denver Post newspaper he was so quiet he would not acknowledge people in his block of flats. He was known as "Jimmy", a cross-country runner and soccer player who quit the team in his mid-teens to focus on his studies (year book photo, bottom right).

Christine Mai, 17, who lived next door to the Holmes' family for 15 years, said she never saw "Jimmy" act violently or carry a weapon. He didn't appear to socialise with friends or bring girlfriends home.

Holmes earned a neuroscience degree from the University of California Riverside in 2010 and graduated with high honours.

"Academically, he was the top of the top," University Chancellor Timothy White told reporters.

He had trouble finding work after graduating and neighbours said he took a job at a nearby McDonalds.

The move to Colorado

Holmes moved to University of Colorado for his graduate courses but was in the process of dropping out, according to the university. Colleagues said he appeared to be influenced by movies and played online role-playing video games.

"I did not see any behaviour he exhibited that indicated he would be capable of an atrocity of a magnitude like this," said Billy Kromka, a pre-med student who served as a research assistant alongside Holmes for several months last year.

"He basically was socially awkward but not to the degree that would warrant suspicion of mass murder or any atrocity of this magnitude," Mr Kromka said, adding that Holmes never talked about politics or became animated about any particular subject.

Shooting spree

Holmes is believed to have bought a cinema ticket, entered the theatre and propped open the emergency exit so he could slip out and return armed in the early hours on Friday, a law enforcement official said.

Theatre-goers called police when the shooting started and officers arrived in less than two minutes. They took Holmes into custody in a parking lot behind the cinema. He surrendered without a fight, Aurora Police Chief Dan Oates said.

In addition to the 12 dead, police said 58 were injured and they described scenes of horror in the cinema, which was screening the film The Dark Knight Rises. Some of the injured were in critical condition, raising the possibility that there are more deaths, shock and grief still to come.

Among the dead were a six-year-old, two servicemen, a 23-year-old community college student, and a sports blogger who narrowly avoided a shooting spree at a Toronto shopping mall in June.

Booby-trapped home

Police tried to enter Holmes' apartment on Friday following the arrest but said he had booby-trapped it with what appeared to be explosives.

Several trip wires were attached to one-litre plastic bottles containing an unknown substance in a sophisticated manner, Police Chief Oates told the Denver Post.

On Saturday morning in Colorado, police placed a remote device on a rooftop across from the building where Mr Holmes lived in Aurora. The plan was to send in a robot to detonate any booby-trapped device in the flat.

Techno music

Kaitlyn Fonzi, a 20-year-old biology student at University of Colorado Denver lives in an apartment below Holmes. She heard techno music blasting from Holmes' apartment around midnight when Holmes is believed to have been at the theatre. Ms Fonzi told reporters she knocked on the door, realised it was unlocked but did not go inside.

At almost exactly 1am, Ms Fonzi said the music stopped.

Holmes' family issued a statement of sympathy for the victims, saying, "Our heart goes out" to their loved ones, while they also asked for privacy from the media while they "process this information."

Police will not say what, if anything, Holmes said after his arrest. On Monday, Holmes is due to make his initial court appearance.

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