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AURORA, Colorado : Police and FBI were questioning the suspected gunman in a shooting early Friday at a crowded midnight screening of the new Batman movie, "The Dark Knight Rises," in Aurora, which left 12 people dead and 59 others wounded.

The 24-year-old man believed responsible is a graduate school dropout who grew up in San Diego, California, and had only one known brush with the law - a traffic ticket last year.

Authorities were "confident" James Egan Holmes, who was clad in all black clothing and wore body armour, acted alone.

He used an AR-15 assault rifle, a shotgun and a .40-calibre Glock handgun in the attack. An identical handgun was also found in his car, which was parked outside the theatre.

Police say Holmes burst into the crowded Century 16 movie theatres in Aurora, Colorado, where midnight showings of the Batman movie "The Dark Knight Returns" were airing in five cinemas and opened fire with three guns, killing at least 12 and injuring scores. Police said early Saturday (Thailand time) at least 59 people were wounded by gunfire, with several in critical condition at Denver-area hospitals.

What snapped in the neuroscience student's brain is unclear. An Aurora man who told The Associated Press he had a beer Tuesday with a stranger he later realised was Holmes said they talked about Peyton Manning, the Denver Broncos' new US football quarterback.

"We just talked about football," said Jackie Mitchell, a furniture mover. "He had a backpack and geeky classes and seemed like a real intelligent guy and I figured he was one of the college students."

Mitchell also said Holmes seemed smart, with a "swagger."

Holmes, who stands 190.5cm (6-foot-3), graduated from San Diego's Westfield High School in 2006, Poway Unified School District confirmed. A gifted student, he studied neuroscience at nearby University of California-Riverside, where he graduated in 2010.

Anthony Mai, a 16-year-old who grew up next door to Holmes, described Holmes as a "solitary" person who largely kept to himself but his behaviour was nothing out of the ordinary.

"He felt a little bit concealed, but it wasn't too much. It was all right" he said. "This is just a feeling in my gut, but I felt like he had something, like he was being picked on or something."

His father, Tom Mai, a retired electrical engineer, said Holmes was quiet.

"I said hello to him once in a while. He seemed to be a shy guy," he told reporters.

He said the Holmes have lived at the San Diego home about 10 years and they were a "very, very nice family." Holmes mother is a nurse. The suspect also has a younger sister.