CENTENNIAL (Colorado) • The gunman who stormed a Batman movie premiere and killed 12 cinema-goers in 2012 has escaped the death penalty, but will spend the rest of his life behind bars.

A Colorado jury last Friday failed to unanimously agree on execution for 27-year-old former graduate student James Holmes, obliging the judge to impose a sentence of life without parole.

Last month, the killer had been convicted on 12 counts of murder in the first degree and scores more charges, including murder, attempted murder and explosives possession. But the defence counsel argued that Holmes has a mental illness and urged jurors to show clemency, an appeal apparently heeded by at least one of the panel of nine women and three men.

On each of the 12 murder counts that could have merited the death penalty, the jury said in a statement read to the court: "We do not have a unanimous final sentencing verdict on this count."

District Judge Carlos Samour thanked jurors for their service and set Aug 24-26 as the dates for Holmes' formal sentencing.

On July 20, 2012, Holmes attacked the packed premiere of The Dark Knight Rises at the Century 16 theatre in Aurora, spraying bullets into the dark auditorium.

Clad in body armour and with his hair dyed a peculiar orange, he fired hundreds of rounds before police halted a spree that left 12 people dead, including a six-year-old child.

Last month, he was convicted on 165 charges, with the jury rejecting the defence's argument that Holmes was not guilty because of his mental illness.

Mr Robert Sullivan, grandfather of the youngest victim, Veronica Moser-Sullivan, criticised the jury. "They didn't buy his sanity... and then they bailed at the end. No, I'm sorry," Mr Sullivan said. "It's not justice. Our loved ones are still gone."

The prosecution had argued that Holmes should be executed by lethal injection. "He picked the time, manner and method of their deaths. Does he deserve a life sentence for that?" district attorney George Brauchler said in his closing arguments last Thursday. "This is about justice."

But assistant public defender Tamar Brady disagreed, arguing before the jurors began their deliberations that "justice without mercy is raw vengeance".

Twice previously in the 15-week trial, jurors had rejected the mental illness defence. First they found Holmes guilty rather than not guilty by reason of insanity. Then they found that mental illness was not a mitigating factor in the shootings. But finally they could not agree on a sentence.

Holmes, who was wearing khaki pants and a blue shirt in court, looked calm and had his hands in his pockets. He showed no reaction to the verdicts.

AGENCE FRANCE-PRESSE