CENTENNIAL, Colo. — Sentencing someone to death in Colorado is harder than you may think, as proven by the lengthy process involved in capital punishment cases.

It’s a process that thousands of potential jurors in the Aurora theater shooting trial have been getting an education in over the course of the last three months.

Usually in a first-degree murder trial, if the jury returns a guilty verdict, the judge must return a sentence of life imprisonment without the possibility of parole. However, in cases like the theater shooting trial, when the prosecution seeks the death penalty, as the Arapahoe County District Attorney is doing in this case, it’s the jury that decides the sentence.

This is why jury selection on capital punishment cases takes so long: The court has to ensure each and every juror can be fair and impartial throughout both the trial and the sentencing hearing, if there is a sentencing hearing. Following a guilty verdict, jurors have to be able to consider both penalties – life imprisonment and death by lethal injection – and follow a specific process to decide the appropriate punishment.

RELATED: Complete theater shooting trial coverage

Capital punishment sentencing hearing phases

A sentencing hearing consists of three phases. In phase one, the prosecution must prove beyond a reasonable doubt at least one statutory aggravating factor, or a fact or circumstance which makes a murder more serious than others.

There are 17 aggravating factors in Colorado, including killing a police officer, killing a judge, an ambush, killing a child, and killing multiple victims, the latter of the two will in all likelihood be presented in the Aurora theater shooting case should it continue to the sentencing phase, as 12 people were killed in the shooting, including Veronica Moser-Sullivan, a 6-year-old child.

In phase two, jurors must consider mitigation, or any reason that might favor a life sentence over a death sentence. Mitigation can include mental illness, a defendant’s background or upbringing, the age of a defendant and a catch-all category that would detail any and all reasons a defense attorney might believe a jury should consider a life sentence. Jurors have to weigh each mitigating factor against any aggravating factors, and only if they are convinced beyond a reasonable doubt that the mitigation does not outweigh the proven aggravation does the hearing proceed to phase three.

In phase three, each juror is called upon to make a moral evaluation of the defendant’s character and crime. In this phase, each juror is asked to apply his or her individual reasoned, moral judgment as to whether death is the appropriate penalty, beyond a reasonable doubt.

The process is set up in such a way that unless every condition is met and every phase is completed, the jury must return a verdict of life imprisonment without the possibility of parole. The vote must be unanimous to return a death sentence.

And therein lies the heart of jury selection in the theater shooting case.

The defense is searching for that one juror who could vote for life in prison against 11 others who vote for death, and the prosecution is fighting just as hard to eliminate those sorts of jurors.

Lawyers fight for death penalty jurors

When prospective jurors for the theater shooting trial come in for individual questioning, they first watch a video in which Arapahoe District County Judge Carlos Samour discusses the aforementioned three phases of a sentencing hearing. He follows up with them as a group, making additional remarks to put the discussion in perspective. Then, the jurors are each questioned by the judge and both counsels to ensure they both understand the process and can be fair and impartial.

To say it’s thorough might be an understatement.

Jurors must be willing and able to consider and decide the facts, fairly and impartially. Unlike following traffic laws, it’s completely allowed — encouraged, in fact — for jurors to be honest and say they can’t follow the law in being fair and impartial. They may just be better suited for a different kind of case, Samour says.

Having a bias in favor of or against the death penalty doesn’t necessarily disqualify a juror, though. The standard Colorado law applies is whether a juror’s views on the death penalty prevent or substantially impair the performance of his or her duties as a juror in a sentencing hearing.

“To put it even more simply, can you set that bias aside?” Samour says in his group remarks.

Both the defense and the prosecution thoroughly question prospective jurors trying to prove a juror can or cannot set their preformed biases aside. The defense always ensures that jurors understand even though the court is talking about possible sentences, their client, James Holmes, the admitted gunman in the July 2012 theater shooting, is presumed innocent unless proven otherwise.

The prosecution almost always asks jurors if they can sentence “that man” to death, pointing to Holmes and never naming him. That question is always accompanied by another question, with prosecutors asking the juror whether he or she could also sentence Holmes to life in prison.

Jurors weigh in on sentencing process

Most prospective jurors have found the sentencing process to be long and a tad tedious, but necessary. Some have even expressed relief that it’s so difficult to sentence someone to death, saying that it’s refreshing to see that the justice system doesn’t take the death penalty lightly.

“It seems long, but it’s something that needs to be done. Without proper thought, something could go wrong,” said a juror retained Thursday, a foster father of five.

Another juror retained Thursday started sobbing when one of the alleged aggravating factors in this case, the murder of a child, was mentioned. A mother of three, she said it hit close to home but she could still consider both penalties.

“It still doesn’t mean he doesn’t deserve a fighting chance. He’s a human,” she said, referring to Holmes.

Even though the court has taken its time with individual questioning to make sure every juror kept can be fair and impartial, it has still retained jurors at a much faster pace than expected, moving the theater shooting trial date up to April 27 from the projected date in June. Group questioning, the final phase of jury selection, will take place on April 13, possibly spilling over into April 14. At that time, the court will narrow down a pool of 120-130 jurors to just 24.

The trial is expected to last four to five months, Samour said.