(CNN) The gunman who killed 14 students and three adults at a Parkland, Florida, high school in 2018 was "cold, calculative, manipulative and deadly" in carrying out his attack, prosecutor Michael Satz said Monday in a Broward County courtroom.
Three days before the shooting, Nikolas Cruz looked into his cellphone camera and declared, "Hello, my name is Nik. I'm going to be the next school shooter of 2018," according to Satz.
Satz's opening remarks marked the start of the penalty phase of Cruz's criminal trial and come months after he pleaded guilty to 17 counts of murder and 17 counts of attempted murder. Prosecutors have asked a panel of 12 jurors to sentence Cruz to death, while his defense attorneys have asked for a sentence of life in prison without the possibility of parole.
The trial, which is likely to last months, will delve more deeply into Cruz's personal history and will feature accounts from victims' families and those who witnessed the massacre.
Satz said Monday there were seven aggravating factors in the killing weighing in favor of the death penalty, including that the attack disrupted a government function (i.e. school) and that the killings were "especially heinous, atrocious or cruel."
"These aggravating factors far outweigh any mitigating circumstances, anything about the defendant's background, anything about his childhood, anything about his schooling, anything about his mental health, anything about his therapy, anything about his care," Satz said.
The defense elected to delay its opening statements until later in the trial.
Three witnesses who were in school on the day of the shooting testified Monday about what happened on Valentine's Day 2018, when Cruz used a semi-automatic rifle to kill 17 people and injure 17 more at Marjory Stoneman Douglas High School. Their testimony was punctuated by audio of a 911 call from a teacher and disturbing cellphone videos taken inside the school that caused emotional reactions in the courtroom.
In court, Cruz sat silently during the proceedings, wearing large glasses, a black face mask, a collared shirt and a sweater. At times, he put his head in his hands and looked down or put his head down on folded arms.
The Parkland massacre launched the student-led movement March For Our Lives, which has pushed for gun safety legislation across the country. Still, the US remains gripped by a gun violence epidemic, with more than 350 mass shootings so far this year, according to the Gun Violence Archive, which tracks the incidents in which four or more people are shot, excluding the shooter.
There was the racist attack at a supermarket in Buffalo, New York, in mid-May, followed soon after by the massacre of elementary school students and teachers in Uvalde, Texas. And just this month, a gunman opened fire at a July Fourth parade in a Chicago suburb, killing seven people and wounding dozens.
The prosecution began opening statements with a detailed timeline of the attack. On February 14, 2018, Cruz took an Uber to Marjory Stoneman Douglas High School, walked inside and fatally shot teenage students in the hallway, fired into several classrooms and shot three adults who were trying to protect students from the carnage, Satz said.
The gunman also retraced his steps and shot several of his victims repeatedly. One victim, 14-year-old Peter Wang, was shot 13 times, Satz said.
In all, Cruz fired 139 rounds inside the school, including 70 on the first floor, two in the stairwell, six on the second floor and 61 on the third floor, according to Satz.
Cruz then dropped his rifle and left the school, blending in with the fleeing crowd. He went to a Subway to get an Icee -- even leaving a tip -- and drank it on a bench, Satz said. He was spotted by an officer and arrested about three miles from the school just over an hour after the shooting.
Fourteen of those killed were students: Alyssa Alhadeff, 14; Martin Duque Anguiano, 14; Nicholas Dworet, 17; Jaime Guttenberg, 14; Luke Hoyer, 15; Cara Loughran, 14; Gina Montalto, 14; Joaquin Oliver, 17; Alaina Petty, 14; Meadow Pollack, 18; Helena Ramsay, 17; Alex Schachter, 14; Carmen Schentrup, 16; and Peter Wang, 15.
Geography teacher Scott Beigel, 35; wrestling coach Chris Hixon, 49; and assistant football coach Aaron Feis, 37, also were killed -- each while running toward danger or trying to help students to safety.
Brittany Sinitch, a teacher at Stoneman Douglas in 2018, was the prosecution's first witness and said students were reading Romeo and Juliet and writing romantic poetry on Valentine's Day when they heard gunshots in the hallway.
"They were incredibly loud," she said. "The ones that were outside of my classroom, it's like you could feel it within your body, all throughout your chest."
Students made sure the door was shut, turned the lights off and hid in the corners of the room and behind Sinitch's desk, she said. She called 911, but "they couldn't hear me over the sound of the gunshots," she said. No one in her classroom was shot or wounded. They remained hidden in the room until police came and signaled for them to evacuate.
Danielle Gilbert, a student at Stoneman Douglas at the time, hid inside a classroom, where three people were shot and one was killed. She took several videos of the shooting, and the audio from those were played in court Monday.
In the harrowing audio, gunshots can be heard, one person moans "someone help me," and people can be heard crying and hyperventilating. After several minutes, police arrive at the room and treat the injured.
Dylan Kraemer, another Stoneman Douglas student in 2018, said he was in classroom 1214 in a class titled "History of the Holocaust" when he heard gunshots. They hid in the corners of the room and behind a file cabinet, but the gunman fired a round of shots into the window of the classroom door, killing two people inside.
The videos and audio caused issues and emotional reactions from those in the courtroom.
At least three of the jurors appeared to be uncomfortable while one video was being played.
Tony Montalto, the father of one of the 14 students that were killed that day, put in ear plugs as the videos and audio played. Other victims' family members could be seen embracing each other as the video played.
Another woman in tears and shaking was seen putting her head to her knees with her ears covered. Officers approached her to escort her out as she left visibly distraught.
During Kraemer's testimony, video and audio was played of the attack, causing some family members in the courtroom to close their eyes and turn. Montalto walked out of the courtroom. Someone in the courtroom could be heard yelling, "Shut it off!"
A defendant in Florida deemed guilty of a capital offense undergoes a separate phase of court proceedings to determine the sentence. In the penalty phase, the court reviews the case and the defendant's history to decide whether he or she deserves death or a lesser sentence like life in prison.
The court generally will hear reasons why the defendant should or should not be put to death, known as aggravating factors and mitigating circumstances, respectively.
In Cruz's case, the jury must be unanimous in finding beyond a reasonable doubt at least one aggravating factor exists. If it happens, jurors must then be unanimous in recommending the defendant be put to death, or his sentence would default to life in prison without the possibility of parole. If they recommend death, the judge could choose to follow the recommendation or sentence Cruz to life instead.
The penalty phase could include jurors visiting the scene of the mass shooting, according to court documents. Judge Scherer wrote that a visit would allow the jurors to analyze "several of the aggravators" the state seeks to prove, the documents show.
The building remains intact but has been sealed for the last four years, WPLG has reported.
The court spent weeks whittling down a pool of hundreds of potential jurors to a group of 12 jurors and 10 alternates. Of the 12 jurors, seven are men and five are women. Nine alternates are women, and one is a man.
Several weeks after jury selection began, the process was disrupted when bailiffs on April 27 restrained Cruz against a wall to protect him from people "making a threat," Scherer told Cruz's attorney. A potential juror was "mouthing expletives" at Cruz, prompting other jurors to become "belligerent," said the judge, who dismissed the rest of the prospective jurors who were in the room.
In another instance, about a dozen potential jurors were dismissed when one of them wore a T-shirt referencing the shooting, CNN affiliate WPLG reported. One of Cruz's attorneys took issue with the shirt, which bore Marjory Stoneman Douglas' official colors of silver and burgundy and read, "Teacher Strong #neveragain #msdstrong."
"She obviously did that on purpose to get out of jury selection," the judge said, according to the TV station. Scherer dismissed the rest of the panel.
During jury selection, Cruz's defense attorneys asked the court for a delay, arguing the "wave of emotion" triggered by a spate of recent shootings would undermine his right to a fair trial, court records show. But state Judge Elizabeth Scherer denied the motion, saying shootings in Buffalo and Uvalde had not compromised court proceedings or his ability to get a fair trial.
CORRECTION: An earlier version of this story incorrectly reported Peter Wang's age at the time of his death. He was 15.