CENTENNIAL, Colo. -- Prosecutors have urged jurors to sentence Colorado theatre shooter James Holmes to death, saying he wanted to murder all 400 people at a midnight movie premiere but managed to kill only 12 because his assault rifle jammed.

Jurors are in the early stages of a complicated three-step process to determine whether Holmes should be executed or sentenced to life in prison without parole for the 2012 attack, which also left 70 injured.

They will resume deliberations Thursday over whether prosecutors have proven at least one of several legal factors that would warrant death.

Prosecutor Rich Orman on Wednesday showed jurors photos of each person killed and read their names -- bringing some of their relatives in the courtroom to tears. Holmes deliberately and cruelly killed all of them, including 6-year-old Veronica Moser-Sullivan who "had four gunshot wounds to her little body," Orman said.

The defence offered no counter-argument, effectively conceding that prosecutors had met the first of several requirements for the death penalty: That at least one aggravating factor was present in the massacre. Those factors include the killing of a child, the number of victims involved, and that the killings were particularly heinous.

Orman reminded jurors that Holmes threw tear gas and sprayed so much gunfire that even moviegoers hiding behind seats couldn't avoid being hurt.

"The victims were unaware of any danger, watching a movie, in a theatre, a place of joy and of safety," he said. "The victims died surrounded by screaming, by pain and by anguish."

Jurors went home for the day without deciding whether prosecutors had met their burden.

The jurors have the final say on Holmes' sentence, but they also have a major influence on how the proceedings unfold. After each phase of the process, they meet to decide whether they've heard enough to make a decision. And with no counterpoint from the defence on aggravating factors, jurors immediately began deliberating whether prosecutors had made that part of the case beyond a reasonable doubt.

Sentencing is expected to last a month and could be even more heart-wrenching and polarizing than the 11-week trial that resulted in convictions on murder, attempted murder and other crimes for his July 20, 2012, attack.

Jurors will decide sentences for only the 12 people Holmes' killed. Sentencing is set by law for the attempted-murder convictions stemming from the 70 people injured.

Assuming jurors agree there were aggravating factors, the defence will lead the next phase, trying to show that mitigating factors make it wrong to execute him. They must consider whether the extent of his mental problems outweighs the lifelong suffering Holmes caused by opening fire on the audience in a crowded Batman movie premiere.

Jurors would then deliberate for a second time, to decide whether the mitigating factors so outweigh the aggravating factors that Holmes deserves life without parole rather than execution. If so, the trial ends there, without the death penalty.

If not, the sentencing will move into a third and final phase, in which victims and their relatives would describe the impacts of Holmes' crimes.

Associated Press writer Dan Elliott in Denver contributed to this story