James Holmes' sanity at the time he gunned down 12 people in a Colorado movie theater went on trial Monday, more than 1,000 days after the shooting spree that shocked the nation.

The 27-year-old, onetime University of Colorado neuroscience student does not dispute carrying out the shootings at the midnight screening of the Batman film, “The Dark Knight Rises,” but has pleaded not guilty by reason of insanity. Holmes knew what he was doing during the two-and-a-half months that he plotted the spree and stockpiled weapons, District Attorney George Brauchler told the 12 jurors and 12 alternates who were left from a record jury pool of more than 9,000. Describing Holmes as a superior intellect who felt rejected by the world and harbored a hatred of mankind, Brauchler pledged to bring forward top mental health experts who evaluated Holmes and concluded he was sane at the time of the attack.

“One guy felt as if he had lost his career, lost his love life, lost his purpose,” said Brauchler, who ticked off short bios of each of the 12 murder victims.

Holmes had foreshadowed his attack with previous pledges to kill, and on the night of the spree, had taken steps to escape, including bringing $280 cash, tire-flattening tacks to throw out the window of his car during a pursuit and yet another handgun he did not take into the theater.

“He couldn’t get away very well, he couldn’t drive,” Brauchler said, implying that Holmes’s plan was methodical and not the product of a confused mind. “The best available option was to give up, which he did.”

“It’s the only thing anyone here is talking about.”

- Prof. Nancy Leong, University of Denver College of Law

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Lead defense attorney Daniel King and attorney Katherine Spengler were expected to argue that Holmes was indeed insane at the time of the attack following Brauchler's arguments.

The attack occurred at 12:30 a.m. on July 20, 2012, when the California native burst into the Century 16 theater dressed in black body armor and a gas mask. As stunned moviegoers watched, some reportedly thinking they were witnessing a prank connected to the film, Holmes set off tear gas grenades and began firing into the audience with an arsenal that included a 12-gauge shotgun, a semi-automatic rifle and a Glock 22 40-caliber handgun.

By 12:45, police had arrested Holmes, who was found next to his car which was parked behind the cinema. They later determined Holmes had bought a ticket and sat in the front row before sneaking out an emergency exit 20 minutes into the film. He left the door propped open with a piece of plastic, and returned to wreak carnage.

Holmes pleaded not guilty by reason of insanity to all 166 murder, attempted murder and other charges in the attack. His attorneys maintain he was "in the throes of a psychotic episode when he committed the acts," but prosecutors said the crime was so intentionally heinous it deserves the death penalty. Two court-ordered psychiatric exams that could determine the verdict were withheld from the public.

On Monday, Holmes arrived at court wearing glasses, a blue and white-striped shirt and khaki pants. His hair was closely cropped, and his appearance was in sharp contrast to that of earlier procedures, when he was brought to court in his state-issue orange jumpsuit and wore his beard and hair long and unkempt. He appeared alert as he chatted with his legal team, read documents and even flashed an occasional smile as Judge Carlos Samour spoke to attorneys and ruled on last-minute motions prior to the opening arguments, which were to begin at noon local time. Holmes' parents and uncle were on hand and sat quietly throughout this morning's proceedings.

Samour, a former prosecutor who became a state district court judge in 2007 after working as a private defense attorney, is a native of El Salvador. He took over the case in March, 2013, after the prosecution's decision to seek the death penalty prompted Chief Judge William Sylvester to step down, saying a capital case would take up too much of his time and affect his administrative duties.

Few of the actual details in the case are expected to be disputed at trial. Instead, lawyers for the defense and prosecution will pore over Holmes’ behavior leading up to the rampage, each trying to prove their theory about his mental state.

Holmes’ downward spiral prior to the shooting is well-documented. His graduate studies in neuroscience had fallen apart, and he had told a classmate he wanted to kill people, prosecutors say.  He had 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.

In his apartment, which police found booby-trapped with explosives, he stockpiled weapons, more than 6,000 rounds of ammunition, tear gas grenades and riot gear.

Whether these details prove he was insane or knew exactly what he was doing will be for a jury -- chosen from a record pool of more than 9,000 -- to decide.

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.

University of Denver Sturm College of Law Professor Nancy Leong told FoxNews.com the Holmes case is “the trial of the century” for the Centennial State, and added that all of her students are following it closely.

“It’s the only thing anyone here is talking about,” said Leong, who teaches criminal and constitutional law. “I can’t remember the last time we had a trial this big.”

An interesting aspect of the case that lends itself to law school lessons is the fact that Colorado is rare among states in that the burden in an insanity defense rests with the state, not the defense. Holmes does not have to prove he was insane at the time of the shooting; the state must prove that he wasn’t.

“That could make a difference in jury deliberations,” Leong said.

The case has also captured the attention of survivors and victims’ families. Larry Trujillo told Fox News his daughter, Tayler, was in the theater and managed to escape unharmed.

“I'm here to support my daughter who was in the theater the night of the event,” he said. “The canister was thrown the row in front of her, and a few of her friends were shot right in front of her that night.”

Trujillo said Holmes will face the ultimate judgment someday, and for now, just needs to be locked up for good.

“I don't believe the trial will do anything, because there's a bigger judge after all this,” he said. “So, I would have loved to seen him just get life in prison and move on with our lives.”

Holmes’ mother, Arlene Holmes, is grappling with the same questions that will soon face the jury. In a recently published book of poems and reflections, she asked what happened to her son.

"What the hell happened?" Arlene Holmes writes in one poem. "How can the kid who read about the 'Berenstain Bears' and Jon Stewart's 'Earth' and 'Uncle John's Bathroom Reader,' how could he change?"

The Associated Press contributed to this report