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If the defendant had been convicted at trial, the same jury that handed up the verdict would have been retained for a separate sentencing proceeding. But when a defendant pleads guilty before trial to a capital felony, as Mr. Cruz did, the court must impanel a jury just for sentencing, unless the defendant waives the right to have a jury make the determination.

What the state does

The prosecution’s job now is to convince the jury that there are aggravating factors in the case that would warrant the death penalty. Among the possible aggravating factors listed in the law are:

• That the killings were “especially heinous, atrocious or cruel”;

• That the defendant “knowingly created a great risk of death to many persons”;

• That the defendant killed “in a cold, calculated, and premeditated manner without any pretense of moral or legal justification.”

Prosecutors are expected to present extensive details of the 17 murders and 17 attempted murders at the high school, including hundreds of gruesome photographs and videos. The jury may also tour the school building where the shooting took place.

“They’re going to try to get these jurors to relive what happened to the victims,” said David S. Weinstein, a former prosecutor who is now a defense lawyer. “It’s going to be an emotional roller coaster.”

What the defense does

The defense will try to convince the jury that there are mitigating circumstances that would call for leniency. Under the law, those circumstances could include that the defendant “was under the influence of extreme mental or emotional disturbance” or had a diminished ability to understand whether his actions were criminal, among other factors.

The defense lawyers intend to show that Mr. Cruz, who was 19 at the time of the shooting, struggled with a difficult upbringing and mental health problems and had tried to get treatment. They have requested permission to show jurors a map of his brain, but the judge has yet to decide whether to allow it.

The jury’s task

After hearing the evidence, the jury must first decide whether the state has proved each of its claimed aggravating factors beyond a reasonable doubt. For the defendant to be eligible for the death penalty, the jury must unanimously find at least one of the aggravating factors to be proved.

Next, the jury would consider whether the proven aggravating circumstances are sufficient to warrant a death sentence and outweigh any mitigating factors found to exist, and if they do, whether to recommend a death sentence to the court. To do so, the jury must again be unanimous; otherwise the sentencing recommendation must be for life in prison without possibility of parole.

The court cannot impose a death sentence if the jury has recommended life in prison, but it can set aside a jury’s recommendation of death and impose a life sentence instead.

A strange case

The need for a special sentencing jury is one of several ways the Parkland case is extremely unusual. It is rare for someone as young as Mr. Cruz (he is now 23) to face the death penalty, and even rarer for someone who has committed so deadly a mass shooting to still be alive afterward to face justice.

“In a sense, we are in completely uncharted waters,” said Robert M. Jarvis, a law professor at Nova Southeastern University in Davie, Fla., who tracks mass killings. “You never get these kinds of trials, because the gunman is always dead.”

Patricia Mazzei contributed reporting.