A harrowing video of the Valentine's Day 2018 shooting at a high school in Florida that left 17 people dead was played at the sentencing trial on Monday of the man who admitted to carrying out the massacre.

Nikolas Cruz pleaded guilty in October to 17 counts of murder and 17 counts of attempted murder for the attack at Marjory Stoneman Douglas High School in Parkland, Florida.

A 12-person jury is to decide whether the now 23-year-old should receive the death penalty or a life sentence for what prosecutor Mike Satz called a "cold, calculated, manipulative and deadly" massacre.

"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," Satz said in opening arguments.

A cell phone video recorded by a student, Danielle Gilbert, was played for the jury. The audio was made available to the public gallery and reporters.

Screams, cries and moans were punctuated by multiple shots as students huddled in their classroom trying to seek cover from bullets coming through the door.

"This can't be real," someone was heard whispering.

Gilbert, who broke down in tears as the video was played, said one person was killed in the classroom and three wounded.

Cruz, who was wearing a black Covid mask, covered his face with his hands and stared down at the table in front of him as the video was played.

Several anguished relatives of the victims fled the courtroom while others wept openly and hugged their loved ones.

'Next school shooter'

Satz, the prosecutor, told the jury that three days before the shooting, Cruz made a cellphone video in which he said "I'm going to be the next school shooter of 2018."

"My goal is at least 20 people with an AR-15 and some tracer rounds," Cruz said in the video. "It's going to be a big event and when you see me on the news, you'll know who I am."

Satz said that Cruz, after fleeing the school, ordered a drink at a Subway sandwich shop and then went to a McDonald's, where he asked the brother of a girl he had just shot for a ride.

The boy, who was not aware at the time that Cruz was the assailant, declined. Cruz was arrested shortly afterwards.

The trial in Fort Lauderdale is the rare instance of a mass shooter facing a jury, as they often either take their own lives or are killed by police.

The death penalty requires that the jury be unanimous. Cruz will otherwise be handed life without parole.

The Florida shooting stunned a country accustomed to gun violence and sparked new efforts, led by students from the school itself, to get lawmakers to pass tougher gun control laws.

'March for Our Lives'

Parkland survivors founded "March for Our Lives," organizing a rally that drew hundreds of thousands of people to the nation's capital, Washington, in 2018.

Thousands turned out for demonstrations organized by the group last month following two other mass shootings: one at a Texas elementary school that killed 19 children and two teachers, and another at a New York supermarket that left 10 Black people dead.

Those shootings helped galvanize support for the first significant federal bill on gun safety in decades.

President Joe Biden signed the bill into law in June but it fell far short of steps he had called for, including a ban on assault weapons.

Cruz bought the AR-15 semi-automatic he used in the attack legally, despite having a documented history of mental health problems.

Expelled from school for disciplinary reasons, Cruz was known to be fixated on firearms -- and had been identified as a potential threat to his classmates.

On the day of the attack, he arrived at the school in an Uber, began shooting indiscriminately at students and staff, and fled nine minutes later, leaving behind a scene of carnage.

The Justice Department reached a $127.5 million settlement in March with survivors and relatives of Parkland victims who had accused the FBI of negligence for failing to act on tips received prior to the attack that Cruz was dangerous.