CENTENNIAL, Colo. (AP) — A prosecutor declared Monday that two psychiatric exams found Colorado theater gunman James Holmes to be sane as he meticulously plotted a mass murder, considering a bomb or biological warfare before settling on a shooting so that he could inflict more "collateral damage."

"Meticulous" was the word Holmes used twice during the exams, District Attorney George Brauchler said, marking the start of a long-awaited, lengthy and emotionally wrenching trial to determine if he'll be executed, spend his life in prison, or be committed to an institution as criminally insane.

The former neuroscience student has pleaded not guilty by reason of insanity to murdering 12 people and wounding 70 at a midnight "Batman" premiere nearly three years ago.

Jurors must eventually decide whether he was unable to know right from wrong because of a mental illness or defect when he slipped into the theater, unleashed tear gas and marched up and down the aisles, firing at people who tried to flee.

"Through this door is horror. Through this door are bullets, blood, brains and bodies. Through this door, one guy who thought as if he had lost his career, lost his love life, lost his purpose, came to execute a plan," said Brauchler, standing before a scale model of the theater.

"Four-hundred people came into a boxlike theater to be entertained, and one person came to slaughter them," the prosecutor said.

Holmes is accused of 166 counts of first-degree murder, attempted murder and an explosives offense for the mayhem he caused in suburban Denver on July 20, 2012.

It remains one of America's deadliest shootings, and that Holmes was the lone gunman has never been in doubt. He was arrested at the scene, along with an arsenal of weapons on his body and in his car.

Holmes sat quietly as Brauchler described his emotional rise and fall, interspersing facts about the defendant's life with poignant details about his victims. He said the once-promising doctoral candidate told his ex-girlfriend that he had an "evil" plan "to kill people" but that she dismissed what he said as "theoretical."

Told by school authorities to find a new career, he instead remained in suburban Denver and fumed, Brauchler said.

For weeks thereafter, while turning away offers of help from his parents, friends and a therapist on campus, Holmes bought guns, protective gear and a detonation system to blow up his apartment with, along with earphones to blast techno music so that he wouldn't hear the screams of his victims, Brauchler said.

Dying his long hair a clown-like red "has nothing at all to do with the movie or being the joker," he said. Holmes did that because he wanted to stand out and "be remembered."

"By the time he gets into that theater ... there is not a millimeter of flesh that is not covered by armor or some protective material," the prosecutor said. "He was wearing a kill suit."

Each side was allowed two hours for opening statements, with public defenders Daniel King and Katherine Spengler following his presentation.

Defense lawyers say Holmes was in the grips of a psychotic episode and could not tell right from wrong when he went on the rampage. His parents, Robert and Arlene Holmes, in pleading for his life, have called their son a "human being gripped by a severe mental illness."

Under Colorado law, the burden falls on the state to prove beyond a reasonable doubt that he was "NOT insane," Judge Carlos A. Samour Jr., told the jury. And that depends in part on "a culpable state of mind:" If Holmes acted with deliberation and intent — willfully taking actions that he knew would kill people -- then even if he had mental problems, he should be found guilty of murder, the judge said.

Prosecutors allege that Holmes planned the violence for months, buying a rifle, a shotgun, two pistols, tear gas canisters, body armor, thousands of rounds of ammunition and a chemical stockpile that turned his 800-square-foot apartment into a booby trap that might have caused a conflagration.

"He tried to murder a theater full of people to make himself feel better and because he thought it would increase his self-worth," Brauchler said. "I would like to focus on the victims," he said, but instead he must prove that Holmes was not insane.

The state has already spent millions seeking that verdict, managing an outsized number of victims, witnesses and more than 85,000 pages of evidence. Nearly three years passed hundreds of motions were filed in legal debates over capital punishment and insanity pleas.

Insanity defenses are successful in only 25 percent of felony trials nationally, even less so in homicides.

"Lay people tend to think of people with mental illness as extremely dangerous, and that also influences jurors, especially if someone has killed someone," said Christopher Slobogin, who teaches law and psychiatry at Vanderbilt Law School. "Usually there's evidence of intent and planning that seems to be counterintuitive to the lay view of mental illness."

Winning a trial on mental-health grounds is rare, but then again, so are jury trials for mass shooters. Most are killed by police, kill themselves or plead guilty.

A review of 160 mass shootings found killers went to trial 74 times, and just three were found insane, according to Grant Duwe, a Minnesota corrections official who wrote the book "Mass Murder in the United States: A History."

Just one has won a mental-health case in the last two decades, Duwe said: Michael Hayes, who shot nine people, killing four, in North Carolina in 1988. Based on that, Holmes "faces some pretty long odds," he said.

Brauchler began laying out how the once-promising doctoral candidate in neuroscience plotted and planned for months, amassing guns, ammunition, tear gas grenades and enough chemicals to turn his dingy apartment into a potentially lethal booby trap that could have caused even more carnage.

Holmes was arrested almost immediately, while stripping off his body armor in the parking lot outside the Century 16 movie theater where he replaced Hollywood violence with real human carnage. His victims included two active-duty servicemen, a single mom, a man celebrating his 27th birthday, and an aspiring broadcaster who had survived a mall shooting in Toronto. Several died shielding friends or loved ones.

At 6 years old, the youngest to die was Veronica Moser-Sullivan. Her mother, Ashley Moser, was left paralyzed and lost her unborn child.

"It still doesn't bring him back, but we want justice," said W. David Hoover, who wants to avenge the death of his 18-year-old nephew, A.J. Boik. "Real justice is going to happen when this animal leaves this Earth."