The Parkland high school shooter, 23-year-old Nikolas Cruz, pleaded guilty in October to all charges. Jury selection, the first phase of the trial, began this week.

On February 14, 2018, Cruz opened fire at Marjory Stoneman Douglas High School in Parkland, Florida, killing seventeen people and injuring seventeen others.

According to Reuters, the Parkland case is one of the deadliest mass school shootings in U.S. history to make it to trial. Seven other school shooting perpetrators who killed seventeen or more people were not tried because they died during the event, either killed by police officers or by suicide.

Previous to his act of violence, Cruz had been expelled from the school for threatening people. His social media accounts showed he possessed knives and guns, killed animals, and wrote about wanting to blow the school up.

Per NBC News, Cruz was seen by professionals at the Henderson Behavioral Health facility in 2016 after he posted a video on Snapchat about cutting himself and wanting to purchase guns. The facility did not admit him.

Cruz reportedly had a history of misbehaving at the school, including setting off fire alarms, participating in physical altercations with peers, breaking windows, and bringing things like bullets to campus.

On Valentine’s Day in 2018, Cruz entered the building and began shooting across the hallways and into classrooms. Fourteen students and three staff members died as a result.

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Witness Kelsey Friend spoke of Scott Beigel, a teacher killed in the shooting.

“We heard ‘bop, bop, bop, bop,'” she said, describing the gunshots. “I thought he was behind me, but he was not. He did, unfortunately, pass away.”

Beigel had opened a classroom door for Friend and other students to take cover from the gunfire.

According to The Washington Times, victims’ loved ones were present in the courtroom as jury selection began.

Tony Montalto, the father of 14-year-old victim Gina Montalto, spoke at the hearing.

“I just hope everyone remembers the victims,” he said. “[Cruz] told the world his plans on social media, carried out those plans in a cold and calculated manner, and murdered my beautiful daughter, thirteen of her classmates, and three of her teachers.”

Broward County Circuit Judge Elizabeth Scherer will be selecting and confirming the jury.

Twelve jurors and eight alternatives will be selected. According to court officials, over 1,500 candidates for a juror spot could participate in the preliminary screening process in the coming weeks.

Because Cruz pleaded guilty, the selected jury will only decide whether he will face life in prison without parole or the death penalty.

The defense plans to present evidence that Cruz may have sustained brain damage as a result of his biological mother’s apparent drug and alcohol use throughout her pregnancy, Cruz’s possible history of mental health issues, and allegations that he may have been bullied and sexually abused.

However, a presentation created by MSD Public Safety Commission argues that Cruz had a history of cruel and violent behavior. His adoptive mother, the late Lynda Cruz, was allegedly physically and verbally abused by her son. The presentation claims she had previously stated that Cruz always knew when he was doing something wrong and that he was good at manipulating people into believing he should not be held responsible for his actions.

Over the next few weeks, groups of sixty potential jurors will arrive at the courthouse on Mondays and Wednesdays, four times per day, to go through the vetting process.

As part of the selection process, potential jurors need to affirm to the judge that they will assess the case fairly. After being questioned by the judge, likely candidates are then asked to fill out questionnaires regarding their background and beliefs, among other personal information. Judge Scherer will make her final selections by May 31.

Cruz can be sentenced to the death penalty only by a unanimous vote from the jury. If one or more jurors dissent, he will receive life in prison without parole.