(NEW YORK) -- A jury will soon determine if confessed Parkland school shooter Nikolas Cruz will be sentenced to death.
Opening arguments began Monday in the penalty phase of Cruz's trial. The trial is expected to take several months. At the conclusion, the jury’s decision must be unanimous to sentence him to death.
Prosecutor Mike Satz said in his opening statement: "Three days before the massacre, Cruz made a video saying, 'My name is Nik. I'm going to be the next school shooter of 2018. My goal is at least 20 people with an AR-15 and some tracer rounds. It's gonna be a big event and when you see me on the news you'll know who I am. You're all gonna die. ... I can't wait.'"
Satz described the shooting as a "planned, systematic ... mass murder."
On Feb. 14, 2018, Cruz, then 19, gunned down 14 students and three staff members at his former school, Marjory Stoneman Douglas High School, in South Florida.
Danielle Gilbert, who was a student in Building 12 when the gunfire rang out, took the stand as a prosecution witness Monday.
Gilbert said students dropped to the ground and ran toward their classroom window, but their teacher realized the window was in view of the door, so they all hid near the teacher's desk.
The gunman fired into Gilbert's classroom, killing one student and injuring three, she said.
Gilbert filmed videos of the chaos on her phone, which the prosecution played for the jurors. As the sounds of gunfire, a blaring fire alarm and children screaming and moaning were played in court, Fred Guttenberg, father of 14-year-old Parkland victim Jaime, put his arms around his wife to comfort her, and Gilbert, still on the stand, was overcome with emotion.
Gilbert said she felt they were "sitting ducks" with no way to protect themselves.
On the day of the shooting, Cruz, wearing a backpack and carrying a case with a rifle inside, walked 360 feet from the road to the doors of Building 12, Satz said in his opening statement. Cruz warned a student in the stairwell to leave, saying "something bad is about to happen," after which that student ran to find a campus monitor, Satz said.
The rampage began at 2:21 p.m. as Cruz opened fire on four students standing in a first floor hallway, Satz said.
Seventy shots were fired on the first floor, Satz said.
Satz went on to outline the rest of the timeline, walking the jury through how teachers and students on the second floor heard the gunfire and hid inside their classrooms.
After firing shots on the second floor, Cruz went to the third floor, where among those killed was teacher Scott Beigel, who was shepherding kids to safety in his classroom, Satz said.
A total of 139 rounds were fired in Building 12, Satz said, and Cruz then fled the school by blending in with other students. He was arrested around 3:40 p.m. about 2.9 miles from the school, Satz said.
The defense opted to delay its opening statement to a later date.
Cruz pleaded guilty in October 2021 to 17 counts of first-degree murder and 17 counts of attempted first-degree murder.
Cruz said in court last year, "I am very sorry for what I did and I have to live with it every day."
Cruz, the deadliest mass shooter to ever go before a jury, said in court he believes the victims' families should be the ones to decide whether he gets the death penalty.
Cameron Kasky, a Parkland student in 2018 and now a gun-reform activist, called the death penalty "barbaric" in a tweet Sunday.
The death penalty "will not bring any of the victims back" and will "create a false sense of Justice, which will only come when the gun manufacturers and the politicians who support them are held accountable," Kasky wrote.
The "shooter does not deserve to live, but the U.S. government and our 'justice' system is incapable of responsibly wielding the power to determine value in life," he wrote. "It's the politicians who support the gun lobby that should be held accountable."
Manuel Oliver, whose 17-year-old son Joaquin died in Parkland, told Miami ABC affiliate WPLG, "I think he should die and I think that is not enough. ... Not even the death penalty is enough."
Student Dylan Kraemer testified Monday that he and his classmates ran to a corner of his classroom and knocked down a filing cabinet to crowd behind it.
Gunfire pierced the window of his classroom door, killing two students and wounding several others, Kraemer said.
Kraemer said he saw the shooter through the window, holding a large AR-15-style rifle.
Kraemer said he later checked the pulse of 17-year-old Nicholas Dworet, who died from his injuries.
Brittany Sinitch, a teacher on the first floor of Building 12, also took the stand for the prosecution.
Sinitch recalled hearing "the loudest noise you could possibly imagine."
One student made sure the classroom door was shut and they turned off the lights and hid, Sinitch told the jurors. Sinitch called 911 but she said the emergency officials couldn't hear her over the gunfire.
She said they stayed quiet until she heard officers in the hallway and they were evacuated. No shots were fired into her classroom.
Fred Guttenberg, who has been fighting for gun reform since his daughter was killed in Parkland, tweeted Monday, "One week ago today I was at the @WhiteHouse to celebrate @POTUS signing gun safety legislation. Today, I am at the Courthouse for the start of the penalty phase of the criminal trial of the person who murdered my daughter with an AR 15. This is the reality of gun violence."
"Jaime, I miss you beyond words," he added in another tweet.
Max Schachter, whose 14-year-old son Alex was killed, tweeted Monday morning, "I love you Alex."