CENTENNIAL, Colo. -- Jurors in the Colorado theatre shooting trial declined to rule out the death penalty Monday as they move toward sentencing James Holmes.

The decision clears the way for one last attempt from both sides to sway the jury with gripping testimony from victims about their harm and suffering as well as more appeals to show mercy to the man convicted of murdering 12 people and trying to kill 70 more.

Holmes, who is taking anti-psychotic drugs that dull his reactions, stood as ordered and appeared emotionless as the judge read the decisions. On all 24 murder counts carrying a potential death penalty -- two for each of the 12 people slain -- the jury found that aggravating factors outweighed mitigating factors.

With that, the jury was sent home and told to return Tuesday morning for the final phase. Then, the nine women and three men will finally decide whether the 27-year-old should receive a lethal injection, or spend life in prison without parole.

The same jury rejected the defence claim that mental illness so warped his mind that Holmes could not tell right from wrong when he carried out the theatre attack in the Denver suburb of Aurora on July 20, 2012.

In the first step of Colorado's complicated death sentencing process, prosecutors then argued, and jurors agreed, that capital punishment could be appropriate because Holmes sprang a terrifying and cruel ambush on hundreds of unsuspecting victims.

In the second step, defence lawyers argued that mental illness nevertheless reduced Holmes' "moral culpability," and that his personal history made him worthy of mercy. They said it was schizophrenia not free will that drove Holmes to murder.

Jurors deliberated for less than three hours Thursday and Monday before deciding that his mental problems and the appealing portrait of a younger, kinder man did not outweigh the horrors of the calculated attack on defenceless moviegoers that Holmes waged.

Holmes slipped into the premiere of a Batman movie shortly after midnight and 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 before surrendering meekly to police outside.