The jury was unable to reach a verdict Thursday and will resume their deliberations Monday, a court spokesman said.
The deliberations began after prosecutors argued that the jury should advance death penalty proceedings to its final, third phase under Colorado law. Defense attorneys, however, urged the jury to give Holmes a life sentence and bring the monthslong trial to an end.
Attorneys for Holmes argued that his schizophrenia mitigates the capital crimes that he was convicted of -- 24 counts, to be exact.
The lawyers' arguments came midway through a sentencing hearing that asks the jury to make a milestone decision about how Holmes killed 12 people and wounded 70 more in a movie theater massacre: Did the mitigating factors outweigh the aggravating factors beyond a reasonable doubt?
"Mental Illness can strike like cancer, without regard to your background, without regard to your status in life, without regard to how intelligent you are. And when James Holmes was born, he had this psychotic mental illness in his blood," public defender Tamara Brady told the jury.
Arapahoe County District Attorney George Brauchler downplayed the mitigating factors -- Holmes' mental illness -- and highlighted the aggravating factors.
"Is mental illness going to be a shield here to protect someone who we will see had the ability to make a million decisions and act completely rationally in every other aspect of his life?" the prosecutor told the jury.
"Nobody, nobody in their right mind could plan a massacre of a theater full of human beings, and we should take comfort in that. But not having the same brain that we have does not protect you from the ramifications of those decisions," the prosecutor argued.
Three dozen witnesses testified during this so-called mitigation phase, including James Holmes' parents and sister. For Thursday's closing arguments, Holmes' parents dressed casually in court. His mother, Arlene Holmes, appeared nervous, fidgeting with her hair as the judge read jury instructions.
On Wednesday, the mother gave emotional testimony about how she wasn't aware of the severity of her son's mental illness.
The jury has already found Holmes guilty of 24 capital murder counts, two for each killing. Holmes was also convicted of attempted murder on all of the 140 counts against him for the 70 people wounded in the shooting.
For each of 24 murder counts, the jury must render a verdict on this question: "Does the jury unanimously find beyond a reasonable doubt that the mitigating factors that exist do not out weigh the aggravating factors proven by the prosecution in phase 1 of the sentencing hearing."
If the answer is yes, the sentencing hearing will continue to the third, final phase of death penalty proceedings. The mitigation phase is the second of the three stages.
If answer is no, proceedings will end with respect to this count, and Holmes will receive a life sentence without the possibility of parole.
If the jury is not unanimous on a particular count, Holmes will receive a life sentence without parole.
On Thursday, Holmes, 27, a former doctoral student in neuroscience, was unusually chatty with his lawyers before the jurors were brought into the courtroom to hear arguments.
At least two dozen survivors and relatives of victims, however, mostly filled the half of the gallery reserved for them.
Holmes opened fire upon a crowded movie theater just after midnight on July 20, 2012, as the audience watched the new Batman film "The Dark Knight Rises" in Aurora, Colorado.