Updated: July 18, 2022 6:47 PM EDT | Originally published: July 18, 2022 2:29 PM EDT

Prosecutors recounted the deadly rampage at Marjory Stoneman Douglas High School in harrowing detail on Monday, as the gunman’s sentencing trial began more than four years after 17 people were killed during a 2018 shooting at the Parkland, Fla., school.

“I’m going to speak to you about the unspeakable, about this defendant’s goal-directed, planned, systematic murder—mass murder—of 14 children, an athletic director, a teacher, and a coach,” lead prosecutor Michael Satz told jurors on Monday, describing the gunman as “cold, calculated, manipulative, and deadly.”

Nikolas Cruz, 23, pleaded guilty last year to 17 counts of first-degree murder. Through this trial, a 12-person jury will now determine whether he is sentenced to death or to life in prison without parole. In order for him to receive the death penalty, the jury’s decision must be unanimous.

Read more: The Parkland Gunman’s Sentencing Trial Will Decide If He Gets the Death Penalty

In a crowded courtroom on Monday, Satz rehashed the timeline of events on Feb. 14, 2018, from when Cruz arrived at the school with an AR-15-style rifle and warned a student that “something bad is about to happen,” through when he began firing at students in hallways and classrooms, killing 17 people and wounding 17 others.

Seated in court, victims’ family members shook their heads and wiped away tears as Satz named each victim, described where they were killed and how many times they were shot.

“The murders—all 17—were heinous, atrocious, and cruel,” Satz said. “All 17 were cold, calculated, and premeditated.”

Defense attorneys for Cruz opted not to give an opening statement on Monday and will, instead, open after prosecutors make their case. They’re expected to argue that Cruz struggled with mental health issues, and to focus on mental illness as a mitigating factor in arguing that he should not be executed.

Under Florida law, there are 16 “aggravating factors” that increase the severity of a crime and make a defendant eligible for the death penalty. In order to rule in favor of the death penalty, the jury must unanimously agree that at least one aggravating factor was proved by prosecutors beyond a reasonable doubt. Defendants can present evidence that mitigating circumstances, which could include mental health issues or childhood trauma, mean that capital punishment should not be imposed.

Prosecutors plan to argue that there were seven aggravating factors in this case, including that Cruz “knowingly created great risk of death to many people”; that the murders were “cold, calculated, and premeditated”; that they were “especially heinous, atrocious or cruel”; and that they were done to “disrupt or hinder a governmental function,” such as a public-school education.

“These aggravating factors far outweigh any mitigating circumstances, anything about the defendant’s background, anything about his childhood, anything about his schooling, anything about his mental health,” Satz said.

This is the deadliest mass shooting ever to go before a jury in the U.S., and the first day of the trial was an early sign of the emotional testimony and gruesome evidence that will be presented to the jury during a trial that is expected to last months.

Those whom prosecutors called to testify on Monday included a Marjory Stoneman Douglas teacher and two former students, who were in the building where the shooting took place and described witnessing their classmates get shot, some fatally.

“We were kind of like sitting ducks,” said Danielle Gilbert, who was a junior in an AP psychology class in which multiple students were shot and one was killed. “We had no way to protect ourselves.”

Victims’ family members cried and hugged each other, and some covered their ears or left the courtroom, as prosecutors presented videos captured during the shooting, in which students could be heard screaming amid gunshots and in which one person repeatedly cried out for help.

The Parkland shooting sparked demands for gun-safety legislation that could prevent future school shootings. And this sentencing trial comes shortly after a shooting at Robb Elementary School in Uvalde, Texas, left 21 people dead and renewed calls for action.

For shooting survivors and victims’ family members, the recent spate of mass shootings and the start of this trial have been stark reminders of the devastating threat of gun violence.

Fred Guttenberg, whose daughter Jaime was killed in the Parkland shooting, was at the White House last week as President Joe Biden celebrated the passage of the most significant gun-safety legislation in decades. But on Monday, he was in the courtroom reliving his daughter’s death.

“Today, I am at the Courthouse for the start of the penalty phase of the criminal trial of the person who murdered my daughter with an AR 15,” he said in a tweet. “This is the reality of gun violence.”

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Write to Katie Reilly at Katie.Reilly@time.com.

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