Nikolas Cruz pleaded guilty on Wednesday to 17 counts of murder and 17 counts of attempted murder in the 2018 mass shooting at Marjory Stoneman Douglas High School in Parkland, Florida, the deadliest ever at a U.S. high school.

At a hearing, Cruz, 23, stood hunched over and entered one guilty plea at a time as the judge read off the charges.

His lawyer, David Wheeler, Broward County’s chief assistant public defender, said last week Cruz intended to plead guilty in the Valentine’s Day attack.

Cruz was a 19-year-old expelled student with a history of mental health and behavioral issues at the time of the “cold, calculated and premeditated” killings, Broward County’s State’s Attorney Office said in court documents.

After the pleas were entered, Cruz removed his COVID-19 mask and apologized to his victims.

“I am very sorry for what I did and I have to live with it every day,” he said. “And that if I were to get a second chance I will do everything in my power to try to help others ... I have to live with this every day, and it brings me nightmares that I can’t live with myself sometimes.”

As a prosecutor read a detailed account of the incident, relatives and friends of the victims sat in the courtroom gallery, wiping tears from their eyes and holding each other.

Because prosecutors have vowed to seek the death penalty, his change of plea from not guilty opens the penalty phase.

A jury would decide whether he should be sentenced to life in prison or death.

In Florida, juries determine whether to impose a death sentence.

Broward County Circuit Judge Elizabeth Scherer scheduled jury selection for the penalty phase in the case to begin on Jan. 4.