CENTENNIAL, Colo. >> James Holmes was growing volatile well before he put on a gas mask and body armor, strapped on a rifle, shotgun, pistol and ammunition, and slipped into a midnight premiere of the Batman film “The Dark Knight Rises.”

He was a sought-after neuroscientist-in-training, but he was falling apart. He told a classmate he wanted to kill people, prosecutors say. He fell out of favor with his professors, who suggested he find a new career. He stopped seeing his psychiatrist, then sent her text messages so threatening that she alerted University of Colorado campus police. He even mailed her his journal, in a package with burned $20 bills.

Months before Holmes opened fire on the audience on July 20, 2012, killing 12 and injuring 70 more in one of America’s deadliest mass shootings, the 24-year-old doctoral student was preparing for violence.

He stockpiled weapons, ammunition, tear gas grenades and riot gear, and rigged his apartment to become a potentially lethal booby trap, cranking techno music in an apparent attempt to lure someone into opening his door. One neighbor who came to complain narrowly avoided a fiery explosion by walking away.

Many observers hope Holmes’ death penalty trial beginning Monday will finally show what twisted a seemingly dedicated scholar into a sadistic killer. Prosecutors have suggested he was angry over his academic decline. But anyone looking for a trigger or tipping point with mass killers is usually disappointed, said J. Reid Meloy, a forensic psychiatrist at the University of California, San Diego.

“There’s no such thing as someone snapping,” said Meloy, who is not involved in the Holmes case. “What we know now is that even if a person is psychotic, they can still plan and methodically go about the preparations to carry out a mass murder.”

Mass violence is usually premeditated, following a path that begins with a personal grievance and is complicated by narcissism and paranoia. But only 1 in 5 of these killers is psychotic, Meloy said.

Psychosis is something Holmes knew all about. Before the shooting, he was preparing to give a class presentation on “MicroRNA Biomarkers” that provide a biological basis for psychiatric and neurological disorders.

About the same time, he was amassing deadly firepower: Two Glock pistols. A shotgun. An AR-15 rifle. Boxes upon boxes of ammunition — 6,295 rounds in all.

Police who searched his apartment also found prescription medications for anxiety and depression, 50 cans and bottles of beer, paper shooting targets, and a “Batman” mask.

“He was absolutely out of his mind,” said Denver defense attorney Iris Eytan, who initially represented Holmes but is no longer involved.

She compared Holmes to schizophrenics she has defended: They are erratic and irrational; they hallucinate.

“Look at his eyes, they’re completely dilated,” she said, referencing a mug shot showing Holmes with hair dyed comic-book orange.

“He was under the influence of something, and I believe it was mental illness,” she said.

Prosecutors say the meticulous plotting shows Holmes was deliberate and calculated, and that evidence suggests he knew right from wrong. For example, Holmes searched online for “rational insanity,” and took haunting selfies the night of the shooting, sticking out his tongue and smiling with a Glock under his face.

“He didn’t care who he killed or how many he killed, because he wanted to kill all of them,” Prosecutor Karen Pearson said.

After an emotionally wrenching trial lasting four months or more, the 12 final jurors, chosen from a pool of 9,000, will have to decide whether he was insane at the time.

If so, the 27-year-old inmate with a doughy face and vacant eyes who sits tethered to a courtroom floor will be committed indefinitely to a state psychiatric hospital.

If not, prosecutors will press for the death penalty over life in prison without parole.

Holmes’ former psychiatrist and two court-appointed doctors who spent days interviewing him will likely be asked questions that could explain how a seemingly harmless college student without so much as a traffic ticket on his record could march up and down the aisles of a stadium-style theater, mercilessly shooting down those who tried to flee.

His mother apparently has no idea. He didn’t do drugs or gamble or even stay out late, she says in a recently published book of poems and reflections.

“What the hell happened?” Arlene Holmes writes in one, titled “Home Videos.”

“How can the kid who read about the Berenstain Bears and John Stewart’s Earth and Uncle John’s Bathroom Reader, how could he change?” she asks in another, “Jim’s Room.” ”I leave his room untouched because I need the memories and tangible evidence that he was a good person.”

“People think he is a monster, but he has a disease that changed his brain,” she writes in another poem.

If Holmes’ mother knows more about a mental illness, her book doesn’t say. The family and their lawyers didn’t respond to requests for comment.

Holmes was a shy teen who rarely started conversations but graduated with highest honors from the University of California, Riverside, and then applied for elite doctoral programs. In one essay, he wrote of mentoring a schizophrenic as a summer camp counselor. The boy “vacuumed the ceiling of our cabin. These kids were heavily medicated but this did not solve their problems, only create new ones. … I wanted to help them but couldn’t.”

In another, Holmes hinted at how his own mind worked, writing that he was “fascinated by the complexities of a long lost thought seemingly arising out of nowhere into a stream of awareness. These fascinations likely stemmed from my interest in puzzles and paradoxes as an adolescent and continued through my curiosity in academic research.”

He landed at the University of Colorado’s Anschutz Medical Campus near Denver, one of just six students awarded entry into the exclusive doctoral program in June 2011. He also earned a National Institutes of Health grant for top performers. But by his second semester, his world was crumbling.

His professors sought to keep him out of their labs. He flunked a key oral exam on June 7, and withdrew from the university three days later. He saw his psychiatrist a final time, then pretty much fell out of sight, his former classmates said.

Outside his 800-square-foot apartment in the suburb of Aurora, neighbors recalled seeing him alone, sometimes drinking beers at a nearby bar.

From July 5 to July 18 — two days before the attack — he logged onto a dating website with the handle “classicjimbo.” His profile said he was seeking “casual sex” and asked “Will you visit me in prison?”

When he showed up at the “Batman” movie dressed head to toe in protective gear, some theatergoers assumed he was part of the show.

Then, he tossed tear gas canisters into the crowd and started shooting.