Holmes added the weapon to his already growing arsenal June 7, hours after he took a key oral exam at the college. ABC News station KMGH-TV in Denver reported that he failed the exam. Three days later, he dropped out of the neurosciences program with no explanation.
Holmes, 24, is being held without bond in connection with the shooting, which left 12 people dead and 58 injured July 20 during a midnight screening of "The Dark Knight Rises."
Experts say it's possible Holmes had an underlying mental illness that was triggered by the stress of failure.
"All of those things could actually make dormant schizophrenia come out, and come out relatively quickly," said Marisa Randazzo, a psychologist who studies targeted violence.
Using the kinds of guns Holmes allegedly fired requires training and practice, and law enforcement officials are now trying to figure out where and with whom.
For now, Holmes is being held in the Arapahoe County Jail. Holmes' odd behavior was first seen by the public when he appeared in court Monday looking dazed, alternately bug-eyed and nodding with his eyes closing.
But ABC News has learned that his loopy court appearance was just one of several bizarre behaviors.
In the hours after his arrest Friday for the massacre at the Aurora, Colo., movie theater, Holmes stared at the wall in the Arapahoe Police Headquarters with his eyebrows twitching.
Holmes told police he was the fictitious Batman villain, the Joker, and when cops put evidence bags over his hands to preserve traces of gunpowder residue, he pretended the bags were puppets, law enforcement sources told KMGH-TV.
While Holmes sits in jail, Christian Bale, who plays the hero, Batman, in "The Dark Knight Rises," and his wife made a surprise visit to Aurora, Colo., Tuesday, laying flowers at the makeshift memorial outside the theater. Bale then spent two and a half hours at Medical Center of Aurora meeting seven of the wounded, doctors and first responders.
"It was very, very emotional for them to see him in person, and they all very much enjoyed his visit," said Bill Voloch, the interim president of the facility. "He was amazing. He was kind. He was a true gentleman."
Bale specifically asked the hospital not to alert the media, but it leaked pretty quickly on Twitter when patients started posting pictures online.
Bale released a statement this weekend, saying, "Words cannot express the horror that I feel. I cannot begin to truly understand the pain and grief of the victims and their loved ones, but my heart goes out to them."
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- Christian Bale