Jurors to decide if there will be Florida school shooter Nicholas Cruz sentenced to death or life without parole, are expected to go to trial Wednesday, ending a three-month trial.

24-year-old Cruz pleaded guilty last year to the murder of 14 students and three employees Park zone‘s Marjory Stoneman Douglas High School On February 14, 2018, the court only had to determine his sentence.

Cruz’s mass killing is the deadliest mass shooting ever brought to justice in the United States. Nine other people in the US, who have fatally shot at least 17 people, died during or immediately after their attacks by suicide or police fire. The suspect in the 2019 mass murder of 23 people at a Walmart in El Paso, Texas, is awaiting trial.

Jurors will be sequestered for deliberations that could take hours or days – no one knows. They were ordered to pack for at least two nights.

Here’s a look at the case, how the seven-man, five-woman jury will reach its decision, and what happens next.

WHAT DID CRUZ DO?

Cruz, by his own admission, began thinking about committing a school shooting in high school, about five years before he actually did it. 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 shootings, saying he tried to learn from them. Bought ammo, a vest to wear and a bag to hide. He chose Valentine’s Day to make sure it would never be celebrated at school again.

He took an Uber to the school and arrived about 20 minutes before dismissal. He entered the three-story classroom building, shooting through hallways and classrooms for about seven minutes. He returned to several wounded men to kill them with a second salvo. He then tried to shoot at the fleeing students from a third-story window, but the thick hurricane glass prevented it. He put down his gun and ran away, but was captured about an hour later.

WHAT IT HAPPENED IN COURT?

Chief Prosecutor Mike Satz kept his case simple. He showed video footage of the shooting and gruesome photos from the crime scene and autopsy. Faculty and students testified about watching others die. He led the jury into a fenced-in building that remained stained with blood and bullet holes. Parents and spouses gave tearful and angry statements.

Cruz’s lead attorney, Melissa McNeil, and her team never questioned the horror he caused, but instead focused on their belief that his birth mother’s excessive drinking during pregnancy caused him to develop fetal alcohol syndrome. Their experts said his strange, disturbing and sometimes violent behavior since age 2 had been misdiagnosed as attention-deficit/hyperactivity disorder, meaning he never received proper treatment. This, they said, made his widowed foster mother depressed.

WHAT DOES IT TAKE FOR CRUZ TO RECEIVE THE DEATH PENALTY?

Jurors will vote 17 times — once for each victim. For a jury to recommend the death penalty for a particular victim, they must first unanimously agree that the prosecution proved beyond a reasonable doubt that the killing had at least one aggravating circumstance, as required by Florida law.

That part doesn’t have to be difficult—the listed aggravating circumstances include knowingly creating a great danger of death to a large number of people, committing the murders “with particular heinousness, particular cruelty or brutality,” or committing them in a “cold, calculated, and deliberate manner.” They must then unanimously agree that the aggravating circumstances warrant the death penalty.

They must then determine whether the aggravating circumstances “weigh” the mitigating factors the defense argued, such as his birth mother’s alcoholism, his adoptive mother’s alleged failure to provide him with adequate psychiatric care, and his guilty plea.

In such a case, the jury may recommend the death penalty, but it does not have to. A juror can ignore the weighing and vote for life out of mercy for Cruz.

A recommendation for the death penalty requires the unanimous vote of at least one victim. If one or more jurors vote to sentence all the victims to life, that will be the verdict.

WHAT HAPPENS IF THE JURY RECOMMENDS THE DEATH PENALTY?

District Judge Elizabeth Scherer will schedule a sentencing hearing, likely in a few months. Cruz’s lawyers will have a chance to convince her to overturn the jury’s verdict and impose a life sentence, but that rarely succeeds. If sentenced to death, he would be sent to Florida’s death row while his case is pending. It will be years before he is executed, assuming the death sentence is not overturned and a retrial is required.

WHAT HAPPENS IF THE JURY ENDORSES A LIFE IMPRISONMENT?

If the jury cannot unanimously agree that Cruz should be put to death for at least one victim, he will be sentenced to life in prison without parole — a decision Scherer cannot overturn. She could arraign him immediately or schedule a future hearing.

After sentencing, the Florida Department of Corrections will send him to a maximum security prison where he will become part of the general public. McNeill hinted in her closing remarks that this could be an extremely dangerous place for someone like Cruz.