Theater shooting in Aurora, Colorado

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It was just past midnight and the showing of the summer's most anticipated blockbuster, the final installment of a Batman trilogy that packed movie theaters the nation over, was under way.

But shortly into first screening of "The Dark Knight Rises" at a theater in Aurora, Colo., a scene unfolded more terrifying than anything Hollywood could have scripted.

James Eagan Holmes, now 25, allegedly sneaked into the theater as the movie was playing, set off tear gas and opened fire. By the time he'd finished 12 people lay dead. Fifty-eight others were rushed to area hospitals with gunshot wounds and other injuries.

Holmes was arrested outside the theater clad in gear similar to that worn by SWAT team members responding to the scene. As they investigated further, they found Holmes' apartment booby-trapped with multiple devices and evacuated neighboring residences while they painstakingly disarmed the dwelling.

Investigators would eventually determine Holmes had gone online and purchased more than 6,000 rounds of ammunition to augment four weapons -- a shotgun, a semiautomatic rifle and two Glock handguns, plus body armor.

Two weeks after the shooting Colorado prosecutors charged him in a 147-count criminal indictment. It was then the world got its first glimpse of Holmes, with hair dyed orange emulating the comic book villain The Joker from the very Batman series prosecutors said he'd turned into real-life terror.

Holmes had enrolled as a graduate student at the University of Colorado Denver in neuroscience after getting his bachelor's degree in his native California. But he was in the process of withdrawing from the program.

Initial attempts to ascribe a motive were difficult. Holmes left nothing overt by way of an explanation for the rampage and has said nothing publicly since his arrest, mirroring the loner image many who knew him described in the months leading up to the Aurora shooting. He also had virtually no digital footprint -- no Facebook page, no blog or Twitter account -- perhaps the best evidence of isolation by a college student in the digital age.

The New York Times more than a month after the shooting published interviews with several former classmates who said they recalled Holmes as intensely shy and equally smart. He displayed the occasional wry wit, but hints about his darker side were left.

He sent a text message to one unidentified student weeks before his rampage, asking her if she'd heard of the psychological condition "dysphoric mania" and warned her to stay away "because I am bad news."

Holmes had been under treatment with a school psychiatrist, Dr. Lynne Fenton. University officials confirmed Holmes had been in treatment, but have not said why, except that his case did not rise to the level of involuntary institutionalization.

The school said Fenton told university police she thought Holmes could be a danger to himself or others and asked officers to check if he had a criminal history but prior to the theater shooting for which he's charged, Holmes never had anything worse than a traffic ticket.

University of Colorado officials also have refused to turn over a notebook Holmes kept as part of his treatment to prosecutors, who unsuccessfully sought a court order to get it. He mailed it to Fenton hours before the shooting.

As the semester ended in June, Holmes showed further signs of deterioration. The student initially described as extremely intelligent did poorly on oral examinations and professors suggested he pursue another course of study. It appeared as though he'd obliged, taking steps to withdraw from the elite program.

Holmes' defense lawyers have hinted they will mount an insanity defense. Colorado is one of just a few states where the burden of proof in a not-guilty-by-reason-of-insanity plea rests with prosecutors, not defense attorneys.

If Holmes does enter that plea, his diary will be turned over to prosecutors as evidence, they said.