Nikolas Cruz pleaded guilty Wednesday to murdering 17 people during a rampage at his former high school in Parkland, Fla., leaving a jury to decide whether he will be executed for one of the nation's deadliest school shootings.

Relatives of the victims who sat in the courtroom and watched the hearing via Zoom shook their heads or broke down in tears as Cruz entered his pleas and later apologized for his crimes.

"Today we saw a cold and calculating killer confess to the murder of my daughter Gina and 16 other innocent victims at their school," Tony Montalto said. "His guilty pleas are the first step in the judicial process but there is no change for my family. Our bright, beautiful, and beloved daughter Gina is gone while her killer still enjoys the blessing of life in prison."

The guilty pleas will set the stage for a penalty trial in which 12 jurors will determine whether Cruz, 23, should be sentenced to death or life in prison without parole. Given the case's notoriety, Circuit Judge Elizabeth Scherer plans to screen thousands of prospective jurors. Jury selection is scheduled to begin on Jan. 4.

Cruz entered his pleas after answering a long list of questions from Scherer aimed at confirming his mental competency. He was charged with 17 counts of murder and 17 counts of attempted first-degree murder for those wounded in the Feb. 14, 2018, attack at Marjory Stoneman Douglas High School in Parkland, located just outside Fort Lauderdale.

As several parents shook their heads, Cruz apologized, saying, "I'm very sorry for what I did.... I can't live with myself sometimes." He also said that he wished it was up to the survivors to determine whether he lived or died.

Anthony Borges, a former Stoneman Douglas student who was shot five times, told reporters after the hearing that he accepted Cruz's apology, but noted that it was not up to him to decide the confessed murderer's fate.

"He made a decision to shoot the school," Borges said. "I am not God to make the decision to kill him or not. That's not my decision. My decision is to be a better person and to change the world for every kid. I don't want this to happen to anybody again. It hurts. It hurts. It really hurts. So, I am just going to keep going. That's it."

Cruz's attorneys announced his intention to plead guilty during a hearing last week.

Following the pleas Wednesday, former Broward County state attorney Mike Satz recounted the details of the murders. Cruz killed 14 students and three staff members on Valentine's Day 2018 during a seven-minute rampage through a three-story building at Stoneman Douglas, investigators said. They said he shot victims in the hallways and in classrooms with an AR-15 semiautomatic rifle. Cruz had been expelled from Stoneman Douglas a year earlier after a history of threatening, frightening, unusual and sometimes violent behaviour that dated back to preschool.

The shootings caused some Stoneman Douglas students to launch the March for Our Lives movement, which pushes for stronger gun restrictions nationally.

Since days after the shooting, Cruz's attorneys had offered to have him plead guilty in exchange for a life sentence, saying that would spare the community the emotional turmoil of reliving the attack at trial. But Satz rejected the offer, saying Cruz deserved a death sentence, and appointed himself lead prosecutor. Satz, 79, stepped down as state attorney in January after 44 years, but remains Cruz's chief prosecutor.

His successor, Harold Pryor, is opposed to the death penalty but has said he will follow the law. Like Satz, he never accepted the defence offer — as an elected official, that would have been difficult, even in liberal Broward County, where Democrats outnumber Republicans by more than 2 to 1.

By having Cruz plead guilty, his attorneys will be able to argue during the penalty hearing that he took responsibility for his actions.

As at any trial, prosecutors will present evidence of the shooting, including security video that reportedly shows many of the killings in graphic detail. They will also be allowed to show evidence that Cruz had long planned the attack and made threats through cellphone videos. There will be testimony from students and teachers who were in the building, including some who were wounded.

Prosecutors will also present testimony from the victims' parents and spouses to demonstrate the toll the deaths have had on families and the community.

The defence will then present mitigating evidence that will likely include testimony about Cruz's life, including his long
history of mental and emotional instability, his father's death when he was five and his mother's death four months before the shootings, when he was 19.

To impose a death sentence, all 12 jurors must agree. If they do, Judge Scherer will make the final decision.