FORT LAUDERDALE, Fla. >> Jurors in the penalty trial of Florida school shooter Nikolas Cruz viewed graphic video today of him murdering 17 people as he stalked through a three-story classroom building at Parkland’s Marjory Stoneman Douglas High School four years ago.

The video, compiled from 13 security cameras inside the building, was not shown to the gallery, where parents of many of the victims sat. Shown later to reporters, it depicts Cruz crouching and stalking, firing at anything that moves, down the halls and into classrooms.

He shoots many of his victims at point-blank range, going back to some as they lay wounded on the floor to kill them with a second volley of shots. In one segment, athletic director Chris Hixon burst through a door to confront Cruz, but was wounded and fell. He crawled behind a pillar. Cruz kills him with a blast as he passes.

The 12 jurors and 10 alternates stared intently at their video screens as it played. Many held hands to their faces as they viewed the 15-minute recording, which has no sound.

Some squirmed. One juror looked at the screen, looked up at Cruz with his eyes wide and then returned to the video.

Cruz did not appear to watch the video, exchanging occasional whispers with one of his attorneys.

The video was played over the objection of Cruz’s attorneys, who argued that any evidentiary value it has is outweighed by the emotions it would raise in the jurors. They argued that witness statements of what happened would be sufficient.

Circuit Judge Elizabeth Scherer dismissed the objection, saying a video that accurately reflects Cruz’s crimes does not unfairly prejudice his case. Prosecutors are using the video to prove several aggravating factors, including that Cruz acted in a cold, calculated and cruel manner.

Cruz, 23, pleaded guilty in October to 17 counts of first-degree murder for the Feb. 14, 2018, massacre. The jury must decide if he should be sentenced to death or life without parole for the nation’s deadliest mass shooting to go before a jury.

Later, jurors heard testimony from Christopher McKenna, who was a freshman. He had left his English class to use the bathroom. He exchanged greetings with two students, Luke Hoyer and Martin Duque, as they crossed paths in the first-floor hallway.

McKenna then entered a stairwell and encountered Cruz assembling his AR-15 semiautomatic rifle.

Cruz, who had been expelled from Stoneman Douglas a year earlier, told McKenna, “Get out of here. Things are about to get bad.”

The video shows Cruz opened fire from behind Hoyer, 15, and Duque, 14. They tried to run into their classroom, but the door was locked. Student Ana Martins testified she went to open the door and could see them through the glass.

“They were scared,” she said. A friend pulled her away before she could open the door and Cruz killed the boys and 14-year-old Gina Montalto, who had been studying there.

McKenna sprinted to the parking lot and alerted Aaron Feis, an assistant football coach who doubled as a security guard.

Feis drove his golf cart to the building, but was fatally shot as he opened the door. Cruz then continued through the second floor, where he fired into classrooms but hit no one.

When he reached the third-floor, the video shows, Cruz found students and teachers in the hallway, preparing to evacuate as the first-floor shots had set off the fire alarm. He fired at them as they ran away. Two girls, 18-year-old Meadow Pollack and 14-year-old Cara Loughran, fell wounded. Cruz shot them again as he past, killing them.

Peter Wang, 15, fell mortally wounded by the door. Jaime Guttenberg, 14, made it through the stairwell door before she fell. A teacher hiding behind the door poked her to see if she was still alive, but she didn’t move.

Cruz would soon run past their bodies, out onto the athletic fields and mingle with the fleeing students, speeding past two girls carrying Valentine’s Day balloons. He would be captured about an hour later in a neighborhood 3 miles (5 kilometers) away.

The jurors also heard testimony from English teacher Dara Hass, who had three students killed and several wounded in her classroom when Cruz fired through a window in the door.

“The sound was so loud. The students were screaming,” said Hass, who wept and dabbed her eyes with tissue as she testified. She thought it might be a drill, but then she spotted the body of 14-year-old Alex Schachter, who had been fatally shot at his desk.

“That’s when I saw it wasn’t a drill,” she said. Two 14-year-old girls also died in the classroom: Alaina Petty and Alyssa Alhadeff.

When police arrived and evacuated her students, Hass said she did not want to leave but officers convinced her.

“I wanted to stay with the students who couldn’t go,” she said, referring to Schachter, Petty and Alhadeff.

One student in her class, Alexander Dworet, said he originally thought the loud bangs were the school’s marching band, but then he felt a “hot sensation” on the back of his head where he had been grazed by a bullet and “I realized I was in danger.” He and other students scrambled away from the window, using Hass’ desk as a barrier.

Dworet’s 17-year-old brother, Nick, was across the hall in his Holocaust studies class. Cruz fired into that classroom, too, killing him.