A US jury on Thursday rejected the death penalty for Nikolas Cruz, who shot and killed 17 people at his former Florida high school, opting instead for life imprisonment without the chance of parole.

As the verdict was read, Cruz, wearing a striped sweater and large glasses, stared down expressionless at the defence table while several relatives of the victims in the public gallery shook their heads in disbelief.

The jury deliberated for a full day on Wednesday and briefly on Thursday before deciding that the 24-year-old Cruz should receive life in prison for the February 2018 murders of 14 students and three staff members at Marjory Stoneman Douglas High School in Parkland, Florida.

A death penalty recommendation needed to be unanimous and one or more of the 12 jurors found it was not justified because of mitigating circumstances.

Prosecutors and Cruz's defence team gave their closing arguments on Tuesday after a three-month trial, during which the jury saw graphic footage of the attack and listened to harrowing testimony from survivors.

The lead prosecutor, Michael Satz, said Cruz, who pleaded guilty to the murders last year, planned and carried out a "systematic massacre" and the appropriate penalty was death.

The 80-year-old Satz, who came out of retirement to try the case, ended his closing arguments by reciting the names of the 17 people who died.

Melisa McNeill, a lawyer for Cruz, urged the jurors to show compassion. She said Cruz was a troubled young man born with fetal alcohol stress disorder to a mother who struggled with homelessness, alcoholism and drug addiction, before putting him up for adoption.

"He was doomed from the womb and in a civilised, humane society, do we kill brain-damaged, mentally ill, broken people?" McNeill asked in her closing statement. "Do we? I hope not."

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

In a matter of nine minutes, he killed 17 people and wounded over a dozen more.

Cruz fled by mixing in with people frantically escaping the gory scene, but was arrested by police shortly after as he walked along the street.

The Justice Department reached a US$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. (AFP)