James Holmes will be sentenced to life imprisonment without the possibility of parole for his Colorado cinema attack after the jury failed to agree on a death penalty.

The jurors had rejected his insanity defence, finding Holmes capable of understanding right from wrong when he murdered 12 people and tried to kill 70 others in 2012.

Prosecutors argued that the former neuroscience graduate student deserved death for methodically planning the massacre.

But the previously decisive nine women and three men did not agree on death for Holmes, whose lawyers blamed the attack on mental illness.

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

He meekly 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 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 victims.

The case could have ended the same way more than two years ago, when Holmes offered to plead guilty if he could avoid the death penalty.

Prosecutors rejected the offer. But the victims and the public might not have ever learned in detail what was behind the shootings had the plea deal been accepted.

The trial - featuring a journal where Holmes had secretly described his murderous plans - provided a rare look inside the mind of a mass shooter.

Most are killed by police, kill themselves or plead guilty. By pleading insanity, he dropped his privacy rights and agreed to be examined by court-ordered psychiatrists. Holmes told one that he had been secretly obsessed with thoughts of killing since he was 10.

His parents testified that he seemed a normal, affectionate child who withdrew socially in adolescence and became fascinated with science but did not seem abnormal.

Holmes studied neuroscience hoping to understand what was happening to his mind. But it was when he moved from San Diego to Colorado to attend graduate school that his meltdown accelerated.

Holmes dropped out of his prestigious doctoral programme at the University of Colorado and broke up with a fellow student, the only girlfriend he had ever had.

He began to buy guns and thousands of rounds of ammunition and scouted out The Century 16 cinema complex to learn which auditorium would offer the highest number of victims.

Holmes also constructed an elaborate booby-trap in his flat a few miles away. It failed to explode, but it was designed to blow up and divert police and firefighters at the precise moment of his calculated attack.

He kept his mounting homicidal thoughts from a university psychiatrist he was seeing. Instead, he described his plans in a notebook that he kept secret until hours before the attack, when he mailed it to the psychiatrist.

In it, Holmes diagnosed himself with a litany of mental problems and methodically laid out his plans, even calculating police response times.

Four mental health experts testified that the shooting would not have happened if Holmes were not severely mentally ill. He was having increasingly palpable delusions that killing others would increase his own self-worth, forensic psychiatrist Jeffrey Metzner said.

Shortly after midnight on July 20, 2012, he slipped into the premiere of The Dark Knight Rises, stood before the capacity crowd of more than 400 people, threw gas canisters, and then opened fire with a shotgun, assault rifle and semi-automatic pistol.