Jurors tasked with deciding if Florida school shooter Nikolas Cruz, who killed 17 people in 2018, will receive the death penalty are expected to begin their deliberations today.

The expelled student was 18 when he legally purchased the AR-15 rifle he used to shoot dead 14 students and three staff members at Stoneman Douglas High School in Parkland on Valentine’s Day.

Lead prosecutor Mike Satz told the jury in Fort Lauderdale that Cruz deserves a death sentence because he “was hunting his victims” as he stalked a three-storey classroom building for seven minutes.

He fired his AR-15-style semiautomatic rifle into some victims at close range and returned to wounded victims as they lay helpless “to finish them off”, the court heard.

He pointed to Cruz’s internet writings and videos, where he talked about his murderous desires and wrote: “No mercy, no questions, double tap. I am going to kill a… ton of people and children.”

Defence attorney Melisa McNeill said neither Cruz nor herself has ever denied what he did and that “he knew right from wrong and he chose wrong”.

But she said the former Stoneman Douglas student is “a broken, brain-damaged, mentally ill young man”, doomed from conception by the heavy drinking and drug use of his birth mother during pregnancy.

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She argued for a sentence of life without parole, assuring them he will never walk free again.

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A large number of the victims’ parents, wives and family members were in court to hear the closing arguments, many of them weeping during the prosecutor’s presentation.

The mother of a murdered 14-year-old girl fled the courtroom before bursting into loud sobs in the hallway.

Cruz’s massacre is the deadliest mass shooting that has ever gone to trial in the US.

The jury will only decide his sentence, and a unanimous vote is required for the death penalty.

Jurors can vote for death if they believe the prosecution’s aggravating factors such as the multiple deaths and the planning outweigh the defence’s mitigating circumstances such as his birth mother’s drinking.

They can also vote for a life sentence out of mercy for Cruz.