Fort Lauderdale (United States) (AFP) - Nikolas Cruz, who shot and killed 17 people at a Florida high school in 2018, planned and carried out a "systematic massacre," a prosecutor arguing for the death penalty said Tuesday.

"What he wanted to do, what his plan was, and what he did, was to murder children at school and their caretakers," prosecutor Michael Satz said in closing arguments at the sentencing trial of 24-year-old Cruz.

"It was calculated. It was purposeful. And it was a systematic massacre," the assistant state attorney said.

"And he picked Valentine's Day to do it," he told a hushed courtroom packed with family members of those gunned down at Marjory Stoneman Douglas High School in Parkland, a town north of Miami.

Cruz pleaded guilty to the shooting and it is now up to a jury to decide whether he receives the death penalty or life in prison.

Satz recounted the day of the massacre in harrowing detail as Cruz stared down at the table in front of him with his head in his hand.

Lawyers defending Cruz will present their final arguments after the state concludes its presentation.

If the jury of seven men and five women does not vote unanimously for capital punishment, Cruz will be sentenced to life in prison with no possibility of parole.

On February 14, 2018, the then 19-year-old Cruz walked into school carrying a high-powered AR-15 rifle. He had been expelled a year earlier for disciplinary reasons.

In a matter of nine minutes, he killed 14 students and three school employees, then fled by mixing in with people frantically escaping the gory scene.

Police arrested Cruz shortly thereafter as he walked along the street.

'Poisoned in the womb'

Melisa McNeill, a lawyer representing Cruz, centered her defense on Cruz's traumatic childhood. She argued that he was born with fetal alcohol stress disorder because his mother, who was homeless, drank heavily while pregnant. She also used drugs.

"He was poisoned in the womb," McNeill told the court back in August. "His brain was irretrievably broken, through no fault of his own."

Cruz's birth mother gave him up in a brokered private adoption, McNeill said, but his adoptive mother also became an alcoholic, and he grew up in a broken home.

Given the challenges he faced, she said, life in prison was a more appropriate punishment than execution.

The shooting stunned the nation and reignited debate on gun control, since Cruz had legally purchased the gun he used, despite his history of mental issues.

On March 24, 2018, nationwide marches inspired by school shooting survivors and parents of victims brought together 1.5 million people -- the largest public turnout ever in defense of stricter gun control laws in America.

But the Parkland shooting prompted no significant reform and gun sales have continued to rise.

There have been more mass shootings, including one in May that left 19 young children and two adults dead at an elementary school in Uvalde, Texas.

After the latest shootings, Congress did pass legislation to increase funding for school security and mental health care.