Old friends of James Eagan Holmes, who police say is responsible for one of the most deadly mass shootings in U.S. history, know him as a friendly jokester who liked to smile.

Holmes, 24, was arrested after allegedly opening fire on a midnight screening of The Dark Knight Rises in Aurora, CO, on Friday, shooting 71 people and killing 12.

But Sumit Shah, an old friend who attended Westview High School in San Diego, CA, with Holmes, described the accused killer as "the funniest, smartest guy" who "always had something witty to say."

"The guy I knew in high school, I don't understand how that could be the same guy," Shah told NBC Bay Area. "He was shy and little quiet, but he was never aggressive or mean. He always had really good grades. He seemed pretty normal."

Another neighbour told NBC 7 San Diego, "He was a nice kid and grew up in a nice neighbourhood. Whenever I spoke to his mother she was very proud of him."

But his mother didn't appear surprised at all when contacted by ABC News for comment on Friday, before police had even informed her of her son's arrest.

"You have the right person," she told ABC. "I need to call the police ... I need to fly out to Colorado."

The family later released a statement via police, asking that people respect their privacy.

"Our hearts go out to those who were involved in this tragedy and to the families and friends of those involved," it reads.

After graduating from high school in San Diego, Holmes appears to have earned a bachelor of science in neurosciences from the University of California, Riverside. He then attended grad school at the University of Colorado School of Medicine in Denver. School officials say he was in the process of dropping out of the neuroscience program, but did not know why.

Meanwhile, cops circled Holmes's Aurora apartment on Friday.

The suspect booby-trapped his apartment with sophisticated explosives, Aurora Police Chief Dan Oates told reporters.

"We are trying to determine how to disarm the flammable or explosive material. We could be here for hours or days," Oates said. "The pictures are fairly disturbing. It looks very sophisticated, how it's booby-trapped. It could be a very long wait."

MSNBC reports that public records show Holmes lived with two roommates at the apartment, which is reserved for students, faculty and staff from the medical campus.

Despite his alleged extensive collection of explosives and weapons, officials say he has no military connection or criminal past.

His only run-in with the law before now was a minor traffic ticket in 2012.

Holmes was scheduled to appear in court to face charges on Monday afternoon.

- With files from Reuters