August 22, 2022 5:17 PM EDT

Attorneys for the man who admitted to killing 17 people at Marjory Stoneman Douglas High School in Parkland, Fla., in 2018 are highlighting his biological mother’s history of drug and alcohol abuse during pregnancy in an attempt to save him from execution.

“He was poisoned in the womb, and because of that, his brain was irretrievably broken through no fault of his own,” Nikolas Cruz’s public defender, Melisa McNeill, said Monday in her opening statement.

Cruz, 23, pleaded guilty last year to 17 counts of first-degree murder. During this trial, a 12-person jury will now determine whether he is sentenced to death, which must be a unanimous decision, or to life in prison without parole.

“Nikolas Cruz’s decision to take an Uber to Marjory Stoneman Douglas High School and kill as many people as he could possibly kill is not where Nikolas Cruz’s story starts,” McNeill told jurors. “Individuals are profoundly impacted by the places that they came from.”

McNeill argued that Cruz’s biological mother abused drugs and alcohol while pregnant with him, causing him to suffer from fetal alcohol spectrum disorder. She said Cruz exhibited developmental delays and behavioral issues as early as preschool, when he struggled to communicate and couldn’t get along with other children. He developed a fascination with firearms, she said, and struggled behaviorally, socially and academically while attending Stoneman Douglas High School until February 2017.

Cruz’s adoptive father died days before he started kindergarten. And his adoptive mother, who McNeill described as “the one consistent person in his life,” died in November 2017.

During the first weeks of the trial, prosecutors and witnesses recounted in harrowing detail the events of the shooting on Feb. 14, 2018, when Cruz killed 17 people and wounded 17 others in an attack that lead prosecutor Michael Satz described as “heinous, atrocious, and cruel.”

Earlier this month, jurors visited the school building, which remains unchanged since the day of the shooting, walking past scattered roses from Valentine’s Day celebrations and unfinished school assignments in classrooms stained with blood and marked by bullet holes. And family members of those who were killed delivered heart-wrenching victim impact statements.

“We will always live with excruciating pain. We have an empty bedroom in our house. There is an empty chair at our dining table,” said Annika Dworet, whose 17-year-old son Nick was killed in the shooting. “We did not get to see Nick graduate from high school or college. We will never see him getting married.”

“We will always hesitate before answering the question, ‘How many kids do you have?’” she added.

This is the deadliest mass shooting to go to trial in the U.S. But prosecutors face the challenge of getting all 12 jurors to agree to sentence Cruz to death. If just one juror is persuaded by defense attorneys’ argument, then Cruz will be sentenced to life in prison.

“It feels like if the death penalty was designed for anyone, it’s someone who’s murdered this many people,” Jillian Peterson, an associate professor of criminology and criminal justice at Hamline University in Minnesota who has studied the backgrounds of mass shooters, told TIME previously. “On the other hand, I think we know from our research that these perpetrators tend to have a very strong mitigation case, in terms of what that pathway to violence looks like and trauma, mental health, suicidality, and all those things.”

Under Florida law, there are 16 “aggravating factors” that increase the severity of a crime and make a defendant eligible for the death penalty. In order to rule for the death penalty, the jury must unanimously agree that at least one aggravating factor was proved by prosecutors beyond a reasonable doubt. Prosecutors argued there were seven aggravating factors in this case, including that Cruz “knowingly created great risk of death to many people”; that the murders were “cold, calculated, and premeditated”; that they were “especially heinous, atrocious or cruel”; and that they were done to “disrupt or hinder a governmental function,” such as a public-school education.

In response, the defense will present evidence about mitigating circumstances, including mental health issues and childhood trauma, that mean the death penalty should not be imposed. On Monday, defense attorneys called Cruz’s biological sister and a woman who knew his biological mother to testify about her drug and alcohol addictions and her use of drugs while pregnant.

“Everyone here agrees that Nikolas deserves to be punished, without a doubt,” McNeill said. “But life without the possibility of parole is a severe enough penalty.”

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Write to Katie Reilly at Katie.Reilly@time.com.

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