CENTENNIAL, United States - Alleged "Batman" theatre gunman James Holmes meticulously planned the 2012 massacre after breaking up with his girlfriend and flunking college, prosecutors said Monday, giving harrowing details of the slaughter.
In opening remarks at Holmes' trial, prosecutor George Brauchler gave the first authoritative blow-by-blow account of the killings that left 12 dead and 70 injured in Aurora, Colorado.
"On a cool July night... 400 people filed into a box-like theatre to be entertained... and one person came in to slaughter them," the prosecutor said, pointing at Holmes in a hushed courtroom.
"One guy who felt as if he had lost his career, lost his love life, lost his purpose, came to execute a plan... He tried to murder a theatre full of people to make himself feel better." The death toll could have been much higher, Brauchler said - but a gun cartridge jammed in the movie theatre, while booby-trapped explosives he rigged at his apartment failed to go off.
The 27-year-old, who has pleaded not guilty due to insanity, listened calmly as the prosecutor set out his case on the first day of his trial, which could see Holmes face the death sentence.
Holmes - who sported neatly-trimmed brown hair and a beard for the first court session - has been in custody since the night of the mass murder in Aurora, Colorado on July 20, 2012.
Grisly and harrowing evidence is being presented at the trial.
In an emergency 911 call from that night, played in court, repeated shots and screams can be heard as a caller tells the police dispatcher about the unfolding massacre.
Brauchler opened his case by showing a photo of the back door of the Aurora movie theatre, smeared with blood.
"Through this door is horror," he said. "Through this door is bullets, blood, brains and bodies." Witnesses said Holmes - who had bright orange hair at the time of the attack - threw smoke bombs before opening fire randomly with weapons including an AR-15 military-style rifle, a 12-gauge shotgun and a .40-calibre pistol.
Prosecutors say Holmes had enough ammunition to kill everyone in the crowded theatre showing "The Dark Knight Rises." Contrary to reports, the shooter's orange hair at the time was not linked to the film's Joker character, Brauchler said.
His apartment was later found to be booby-trapped with an array of homemade explosive devices, which police had to disarm before entering the dwelling.
The prosecutor told how Holmes had been dumped by his first ever girlfriend in the months before the attack, and had also failed to make the grade at college, leaving him with no obvious next step.
"I decided to dedicate my life to killing others," Holmes wrote in a notebook he posted the day before the massacre to a college psychiatrist whom he had seen for counselling in the months before the attack.
The shooter's defence lawyer sought to impress on the jury that his relatives, on both sides of the family, had histories of schizophrenia.
"Treat Mr Holmes with human dignity, as a person suffering from schizophrenia," said lead public defender Daniel King.
The defence will not contest anything that happened, but will focus on Holmes's history of mental illness that he could not control, King said.
"You can come at this from any direction and not come up with any reason that makes sense. It doesn't. There's no logic to it. It's not based on reality," he said.
Proceedings have dragged on for more than two and a half years because the prosecutor is seeking the death penalty, according to Denver attorneys following the case.
The defendant has undergone two psychiatric examinations.
His parents Robert and Arlene Holmes - who were in court - wrote a letter to the editor of The Denver Post in December saying their son had never harmed anyone prior to the shooting.
"He is not a monster. He is a human being gripped by a severe mental illness," the couple wrote of their son, who was a neuroscience graduate student at the University of Colorado School of Medicine before the shooting.