United States theatre gunman James Holmes will be sentenced to life in prison without parole after a jury failed to agree on whether he should get the death penalty for his murderous attack on a packed movie premiere in Colorado in 2012.

The nine women and three men said on Friday that they could not reach a unanimous verdict on each of the murder counts. That automatically eliminates the death penalty for the failed neuroscientist, who blamed his calculated murders of 12 people on mental illness.

Prosecutors argued Holmes deserved to die because he methodically planned the 2012 assault at a midnight screening of a Batman movie, even blasting techno music through ear phones so he wouldn't hear his victims scream.

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The same jury had rejected his insanity defence, finding Holmes capable of understanding right from wrong when he murdered 12 people and tried to kill 70 others.

But the defence countered that his schizophrenia led to a psychotic break, and that powerful delusions drove him to carry out one of the nation's deadliest mass shootings.

There was never any question during the gruelling, four-month trial as to whether Holmes was the killer.

Holmes surrendered outside the theatre, where police found him clad head-to-toe in combat gear.

The trial hinged instead on the question of whether a mentally ill person should be held legally and morally culpable for an act of unspeakable violence.

It took jurors only about 12 hours of deliberations to decide the first part - they rejected his insanity defence and found him guilty of 165 felony counts.

The defence then conceded his guilt, but insisted during the sentencing phase that his crimes were caused by the psychotic breakdown of a mentally ill young man, reducing his moral culpability and making a life sentence appropriate.

The jury's final decision came after days of tearful testimony from relatives of the slain.

Source: AP