FORT LAUDERDALE, Fla. (AP) The trial to determine whether the gunman who attacked a Parkland, Florida, high school is executed began Monday with a prosecutor describing how Nikolas Cruz mowed down his 17 victims, returning to some as they lay wounded to kill them with a second volley.

Some parents wept as prosecutor Mike Satz described in his opening statement how Cruz killed their children at Marjory Stoneman Douglas High School on Feb. 14, 2018. Others sat stoically, their arms crossed over their chests. One woman who lost her daughter fled the courtroom, sobbing and holding tissue to her face.

Satz detailed how Cruz shot each of the 14 students and three staff members who died and some of the 17 who were wounded. Some were shot sitting at their desks, some as they fled, some as they lay wounded on the floor as Cruz methodically stalked through a three-story building for almost seven minutes.

Cruz, 23, pleaded guilty in October to murder and attempted murder and is only contesting his sentence. The trial, which is expected to last four months, was supposed to begin in 2020, but it was delayed by the COVID-19 pandemic and legal fights.

Satz called the murders cold, calculated, cruel and heinous, quoting the video Cruz, then 19, made three days before the shooting.

This is what the defendant said: Hello, my name is Nik. Im going to be the next school shooter of 2018. My goal is at least 20 people with an AR-15 and some tracer rounds. Its going to be a big event, and when you see me on the news, youll know who I am. Youre all going to die. Ah yeah, I cant wait, Satz said.

The seven-man, five-woman jury , backed up by 10 alternates, will decide only if Cruz is executed or is sentenced to life without parole.

It is the nations deadliest mass shooting to go before a jury.

Nine other gunmen who killed at least 17 people died during or immediately after their shootings, either by suicide or police gunfire. The suspect in the 2019 slaying of 23 people at a Walmart in El Paso, Texas, is awaiting trial.

It wasnt clear if anyone was present in the courtroom to support Cruz, who sat at the defense table between his attorneys. He mostly looked down at a pad of paper with a pencil in his hand, but he did not appear to write. He would sometimes look up to stare at Satz or the jury, peer at the audience or whisper to his lawyers.

After Satz spoke, Cruzs lawyers announced that they would not give their opening statement until it is time to present their case weeks from now. That is a rare and risky strategy because it gives Satz the only say before jurors examine grisly evidence and hear testimony from survivors and the victims parents and spouses.

When lead defender Melisa McNeill gives her statement, she will likely emphasize that Cruz is a young adult with lifelong emotional and psychological problems who allegedly suffered from fetal alcohol syndrome and abuse.

The prosecutors first witness will be called on Monday afternoon. They have not said who that will be.

Its the first death penalty trial for Circuit Judge Elizabeth Scherer . When jurors eventually get the case in the fall, they will vote 17 times, once for each of the victims, on whether to recommend capital punishment.

Every vote must be unanimous. A non-unanimous vote for any one of the victims means Cruzs sentence for that person would be life in prison. The jurors are told that to vote for the death penalty, the aggravating circumstances the prosecution has presented for the victim in question must, in their judgment, outweigh mitigating factors presented by the defense.

Regardless of the evidence, any juror can vote for life in prison out of mercy. During jury selection, the panelists said under oath that they are capable of voting for either sentence.