Jurors have determined that Nikolas Cruz should be sentenced to life in prison for the 2018 school shooting in Parkland, Florida, that killed 17 people.
Cruz, 24, had pleaded guilty last year to premeditated murder at Marjory Stoneman Douglas High School.
He used a semi-automatic assault rifle to kill 14 students and three staff in one of the deadliest school shootings in American history.
Jurors determined that in each of the 17 murders that mitigating circumstances outweighed aggravating factors, so the death penalty was not supported.
He will be sentenced next month.
The prosecution during the three-month sentencing trial had argued Cruz's crime was both premeditated as well as heinous and cruel, which are among the criteria that Florida law establishes for deciding on a death sentence.
Cruz's defence team had acknowledged the severity of his crimes, but asked jurors to consider mitigating factors, including life-long mental health disorders resulting from his biological mother's substance abuse during pregnancy.
Under Florida law, a death sentence could only have been handed down if jurors had unanimously recommended he be executed. The only other option was life in prison.
Cruz, who was a 19-year-old expelled former student of the school, had apologised for his crimes and asked to be given a life sentence without the possibility of parole in order to dedicate his life to helping others.
The sentencing proceedings included testimony from survivors of the shooting, as well as mobile phone videos in which terrified students cried for help or spoke in hushed whispers as they hid.
The Parkland shooting had led to renewed calls for tighter gun control in the United States.
US gun violence gained renewed attention following mass shootings this year at a school in Uvalde, Texas, that left 19 children and two teachers dead, and another at a supermarket in Buffalo, New York, that killed ten people.
US President Joe Biden in June signed the first major federal gun reform legislation in three decades, which he called a rare bipartisan achievement.
