killed 12 cinemagoers escaped the death penalty but will likely spend the rest of his life behind bars.

A Colorado jury failed to find unanimity 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 defense counsel argued he 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 had 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 and ... understand the court will impose a sentence of life imprisonment without the possibility of parole."

District Judge Carlos Samour thanked the jury for their service and set August 24 to 26 as the dates for Holmes' formal sentencing.

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

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

Three years later, in July of this year he was convicted on 165 charges, the jury rejecting the defense case that he was not guilty because of his mental illness.

The prosecution argued that he should be executed through 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 Thursday.

"This is about justice."

- 'Easier to kill a monster' -

But Assistant Public Defender Tamar Brady disagreed, arguing before the jurors began their deliberations that "justice without mercy is raw vengeance."

She also objected to Brauchler's characterization of the defendant as "evil."

"It is easier to ask you to kill a monster than to ask you to kill someone who is mentally ill," Brady said. "This tragedy was born of disease."

Twice previously in the 15-week trial jurors had rejected the mental illness defense.

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.

In the closing hours of deliberations the jury reviewed a 45-minute, silent video of the gruesome crime scene -– a theater auditorium littered with bullets, bodies, popcorn, blood and gore.

But finally they were unable to agree a sentence.

After a guilty verdict in Colorado death penalty cases, jurors are asked to deliberate three times as to whether the appropriate punishment is death or life in prison without the possibility of parole.

During the guilt or innocence part of the trial, jurors heard from several of the 70 survivors who were injured in the shootings.

Holmes, was also found guilty on 140 counts of attempted murder and will be sentenced on those charges at a later date.

Colorado has executed a prisoner by lethal injection only once since 1977, rapist and murderer Gary Lee Davis in 1997.

There are currently three people on death row in the state, amid signs that the official mood is moving against capital punishment.

In May 2013, Colorado's Governor John Hickenlooper said it was unlikely he would ever allow the execution of one of those, convicted killer Nathan Dunlap.

Hickenlooper granted Dunlap an indefinite reprieve, citing doubts about the fairness of Colorado's death penalty.