FORT LAUDERDALE, Fla. — The jurors who will decide whether Florida school shooter Nikolas Cruz is sentenced to death or life without parole are expected to begin their deliberations Wednesday, concluding a three-month trial.

Cruz, 24, pleaded guilty last year to the murders of 14 students and three staff members at Parkland’s Marjory Stoneman Douglas High School on Feb. 14, 2018. The trial has only been to determine his sentence.

Cruz’s massacre is the deadliest mass shooting that has ever gone to trial in the U.S. Nine other people in the U.S. who fatally shot at least 17 people died during or immediately after their attacks by suicide or police gunfire. The suspect in the 2019 massacre of 23 at an El Paso, Texas, Walmart is awaiting trial.

The jurors will be sequestered during their deliberations, which could take hours or days — no one knows. They have been told to pack for at least two nights.

Here is a look at the case, how the seven-man, five-woman jury will come to their decision and what will happen after that.

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Cruz, by his own admission, began thinking about committing a school shooting while in middle school, about five years before he carried it out. He purchased his AR-15-style semi-automatic rifle almost exactly a year before the shooting and his planning became serious about seven months in advance. He researched previous mass shooters, saying he tried to learn from their experience. He bought ammunition, a vest to carry it and a bag to hide it. He picked Valentine’s Day to make sure it would never be celebrated at the school again.

He took an Uber to the school, arriving about 20 minutes before dismissal. He went inside a three-story classroom building, shooting down the halls and into classrooms for about seven minutes. He returned to some wounded to kill them with a second volley. He then tried to shoot at fleeing students from a third-floor window, but the thick hurricane glass thwarted him. He put down his gun and fled, but was captured about an hour later.

Lead prosecutor Mike Satz kept his case simple. He played security videos of the shooting and showed gruesome crime scene and autopsy photos. Teachers and students testified about watching others die. He took the jury to the fenced-off building, which remains blood-stained and bullet-pocked. Parents and spouses gave tearful and angry statements.

Cruz’s lead attorney Melisa McNeill and her team never questioned the horror he inflicted, but focused on their belief that his birth mother’s heavy drinking during pregnancy left him with fetal alcohol spectrum disorder. Their experts said his bizarre, troubling and sometimes violent behavior starting at age 2 was misdiagnosed as attention-deficit/hyperactivity disorder, meaning he never got the proper treatment. That left his widowed adoptive mother overwhelmed, they said.

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The jurors will be voting 17 times — once for each victim. For the jurors to recommend a death sentence for a specific victim, they first must unanimously agree that the prosecution proved beyond a reasonable doubt that the killing involved at least one aggravating circumstance as proscribed under Florida law.

This part should not be difficult — the listed aggravating circumstances include knowingly creating a great risk of death to numerous people, committing murders that were “especially heinous, atrocious, or cruel” or committed in a “cold, calculated, and premeditated manner.” They then must unanimously agree that the aggravating factors warrant consideration of the death penalty.

They then must determine whether the aggravating circumstances “outweigh” the mitigating factors that the defense argued such as his birth mother’s drinking, his adoptive mother’s alleged failure to get him proper psychiatric care and his admission of guilt.

If they do, the jurors can then recommend a death sentence — but that’s not required. A juror can ignore the weighing exercise and vote for life out of mercy for Cruz.

A death sentence recommendation requires a unanimous vote on at least one victim. If one or more jurors vote for life on all victims, that will be his sentence.

Circuit Judge Elizabeth Scherer will schedule a sentencing hearing, likely months from now. Cruz’s attorneys will have an opportunity to persuade her to override the jury and impose a life sentence, but that rarely succeeds. If sentenced to death, he will be sent to Florida’s Death Row while his case goes through appeals. It will be years before he is executed, assuming the death sentence isn’t overturned and a retrial is required.

If the jury cannot unanimously agree that Cruz should be executed for at least one victim, he will be sentenced to life without parole — Scherer cannot overrule the jury. She could sentence him immediately or schedule a future hearing.

After he is sentenced, the Florida Department of Corrections would assign him to a maximum security prison where he would be part of the general population. McNeill, in her closing argument, alluded that could be an exceedingly dangerous place for someone like Cruz.

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