What We Know About the Parkland School Shooting Case

Follow for live updates on the trial of the Parkland school shooting gunman, who faces the death penalty.

Last fall, the former student who waged a deadly attack against his classmates and teachers in Parkland, Fla., in 2018 pleaded guilty to the premeditated murder of 17 people and the attempted murder of 17 others.

The fate of Nikolas Cruz, now 23, rests with a jury, whose members will decide whether he deserves the death penalty for his crimes or will spend the rest of his life in prison without the possibility of parole. The trial in this penalty phase is set to begin on Monday in a Fort Lauderdale courtroom.

The proceeding is relatively rare: No American mass shooter who has killed as many people as Mr. Cruz has in the modern era has faced trial. The others died during their attacks, either by suicide or in a confrontation with the police.

The shooting at Marjory Stoneman Douglas High School shook the suburban community and inspired a generation of young political activists. Here is what we know about the case.

The gunman had a history of mental health problems.

A deeply troubling portrait of Mr. Cruz began to emerge almost immediately after the police identified him as the suspect in the massacre. Public records, social media posts and interviews with people who knew him revealed that Mr. Cruz, then 19, had a history of mental health and behavioral problems and a fascination with guns. The authorities had missed several opportunities to intervene in his life as it unraveled.

His parents adopted him when his biological mother gave him up after he was born. She played no role in his upbringing, but her extensive criminal and drug abuse history is expected to come up in the trial.

Records showed that his behavior worsened after difficult times in his life. He found his father dead at home of heart disease in 2004. He learned he was adopted when he was 15. His mother died of pneumonia in 2017, just a few months before the shooting.

He had struggled in school since kindergarten and exhibited aggression since middle school. His mother told officials he had been diagnosed with A.D.H.D., autism and obsessive compulsive disorder.

In 2016, he told another student that he had a gun at home and was thinking of using it, prompting two guidance counselors and a sheriff’s deputy to conclude that he should be forcibly committed for psychiatric evaluation. But he never was, despite making threats to himself and others.

He briefly attended an alternative school for students with emotional problems, where he thrived. But he wanted to return to Stoneman Douglas. He did well there for some time, but his behavior eventually worsened and he declined to receive therapeutic care once he turned 18. By February 2017, failing grades forced him to withdraw. Three days later, he bought the semiautomatic rifle he would use in the shooting.

The police were repeatedly called to his residences but said they never found enough reason to arrest him. The F.B.I. mishandled the investigation of tips about his interest in school shootings.

After Mr. Cruz’s mother died, a friend of hers promised to take care of him and his brother. The friend kicked Mr. Cruz out after less than a month, having called 911 on him three times, in large part because he wanted to keep guns. He was living with a schoolmate’s family at the time of the shooting.

Before the massacre, he recorded three videos on his cellphone that indicated that, like other young perpetrators of mass shootings, he wanted to be remembered.

The gunman had a semiautomatic rifle and 300 rounds of ammunition.

The Parkland school shooting occurred near dismissal time on the afternoon of Feb. 14, 2018. Mr. Cruz slipped into the high school through an open pedestrian gate.

He carried a rifle bag stuffed with a semiautomatic rifle and more than 300 rounds of ammunition and walked into Building 12, also known as the freshman building.

Once inside, as he loaded his rifle, he happened across a student. “You better get out of here,” he warned him. “Something bad is about to happen.”

Then he began to shoot. His rampage, which lasted just under six minutes, took the lives of 14 students and three faculty members.

Killed in the shooting were Alyssa Alhadeff, 14; Scott Beigel, 35; Martin Duque, 14; Nicholas Dworet, 17; Aaron Feis, 37; Jaime Guttenberg, 14; Christopher Hixon, 49; Luke Hoyer, 15; Cara Loughran, 14; Gina Montalto, 14; Joaquin Oliver, 17; Alaina Petty, 14; Meadow Pollack, 18; Helena Ramsay, 17; Alex Schachter, 14; Carmen Schentrup, 16; and Peter Wang, 15.

Mr. Cruz killed the 17 victims and injured 17 others without entering a single classroom. Instead, he took aim at students and teachers trapped in the hallways or locked inside classrooms, shooting at them from the door. Several times, he returned to people he had already wounded to shoot them dead.

He left the school undetected and was arrested about an hour later, walking down a residential street.

The shooting began a national gun control movement fueled by student activists. The Florida Legislature adopted some gun restrictions, which Rick Scott, the Republican governor at the time, signed into law.

A state investigative commission found that there were failures on the part of authorities on the day of the shooting, including that eight sheriff’s deputies ignored protocol for active shooters that called for pursuing the gunman to try to disarm him.

A sheriff’s deputy faces criminal charges.

Scot Peterson, the only armed sheriff’s deputy at the school when the massacre began, took cover outside rather than charging inside.

Mr. Peterson, who was fired, was charged with multiple felony counts of child neglect; a trial is scheduled to begin next February. He has said he did not confront the gunman because he did not know where Mr. Cruz was or whether he was firing from inside or outside the building. His lawyers have also argued that he should not be held criminally responsible for any lack of training or clear policy direction from his former boss, Sheriff Scott Israel, whom the state removed from his position following the shooting.

In a sworn deposition last year, Mr. Israel said it was reasonable for Mr. Peterson not to know where the gunman was. “I don’t think at any point he should have known precisely, unless he sees the killer, where the killer is firing from,” Mr. Israel said, according to an excerpt released by Mr. Peterson’s lawyer. Mr. Israel later told The South Florida Sun Sentinel that he still thought Mr. Peterson should have figured out Mr. Cruz’s approximate location and gone into the building to try to disarm him.

Other trials related to the shooting are also pending.

Robert W. Runcie, the schools superintendent in Broward County, Fla., at the time of the Parkland massacre, came under withering condemnation for what critics said was the district’s failure to properly monitor and address Mr. Cruz’s behavior.

Some of the victims’ families tried to force Mr. Runcie’s firing, an effort that failed in a 6-3 vote of the school board in March 2019. Two of the votes against him were cast by victims’ relatives who had been elected to the board after the shootings.

Two years later, Mr. Runcie was arrested as part of a criminal investigation into the shootings and resigned from his $357,600-a-year job. He was charged with felony perjury for lying to a grand jury investigating “possible failures in following school-related safety laws and mismanaging funds solicited for school safety initiatives,” the Florida Department of Law Enforcement announced. Prosecutors say he contacted witnesses to prepare for his grand jury testimony and then lied about it.

The authorities also arrested Barbara J. Myrick, the district’s general counsel, on a felony charge of disclosing secret proceedings of the grand jury investigating the Parkland shootings. Both Mr. Runcie and Ms. Myrick pleaded not guilty to the charges and negotiated handsome exit compensation packages from the school district, his for almost $755,000, hers for $226,349. They have pleaded not guilty and their trials are pending.

Mr. Runcie went on to become the interim leader of Chiefs for Change, an advocacy group whose aim is to improve school safety around the country.

A long trial is expected.

The high-profile trial is expected to be an emotional proceeding that could last several months. Jury selection alone took nearly three months.

Under Florida law, a death sentence would require a unanimous jury. If a single juror disagrees, the gunman would be sentenced to life.