Fort Lauderdale (AFP) - A US jury on Thursday rejected the death penalty for Nicholas Cruz, the shooter who killed 17 people at his former high school in Florida, opting instead for life in prison without the possibility of parole.

The jury deliberated all day Wednesday and briefly Thursday before deciding to sentence Cruz, 24, to life in prison for the February 2018 shooting and death of 14 students and three staff members at Marjory Stoneman Douglas High School in Parkland, Florida.

The death penalty should be recommended unanimously by the panel, in which case at least one or more members of the 12-member panel considered that the penalty was not justified because of mitigating circumstances.

As the verdict was read, Cruz stared blankly at the defense table, while many of the victims' relatives who attended the hearing shook their heads in disbelief.

Cruz had pleaded guilty to the crime he carried out on Valentine's Day last year, and prosecutors said during the three-month sentencing hearings that the appropriate punishment is the death penalty.

Cruz's lawyer, Melissa McNeil, had called on the jury to show clemency to a young man, whom she described as "mentally ill, psychologically ill and a broken person."