Judge Elizabeth Scherer excused the jury for the night after the first day of testimony in Cruz' sentencing trial.

Prosecutors delivered opening arguments Monday before calling seven witnesses, including two former students and a teacher at Marjory Stoneman Douglas High School. They also played harrowing videos from the 2018 massacre for the jurors.

The prosecution expects to call more witnesses on Tuesday, after spending the last hour establishing the chain of custody for video from the school's surveillance system that the FBI obtained.

Attorneys for Nikolas Cruz called for a mistrial, saying their client could not be given fair consideration after the jury witnessed an emotional reaction in the gallery to video of the shooting.

An attorney for Cruz said a family friend of a victim was escorted out of the room after shouting, "Turn it off, turn it off!" during a video of the shooting played as part of ex-student Dylan Kraemer's testimony.

Judge Elizabeth Scherer disagreed, striking down the motion.

Dylan Kraemer, a junior at Marjory Stoneman Douglas High School on the day of the 2018 shooting, recalled the attack from the stand Monday.

"We knocked down a file cabinet, and tried to get as many students behind it [as we could]," he said.

"Eventually the shooter started shooting through the window [in the door] and bullets were flying through," he said. "I looked over and two people were dead and multiple people were shot."

Kraemer described Nikolas Cruz as wearing a hat and a vest, holding "a large AR-15."

Kraemer identified Nicholas Dworet as one of the students killed in his classroom, and said he checked Dworet's pulse after the shooting.

Prosecutors played a video of the events in Kraemer's classroom, which depicted multiple loud gunshots over frightened shouts of "Oh s--t!" and "Oh my God!" and much screaming.

The jury that will decide admitted school shooter Nikolas Cruz's fate was shown harrowing video Monday made by student Danielle Gilbert during the attack.

The video was not visible to the gallery, but people in the courtroom could be seen crying at the audio, which went on for nearly seven minutes and included screams, moans of pain and cries for help after a fusillade of gunshots.

The recording runs from the beginning of the attack through the police evacuation of Gilbert's classroom, where four students were hit, one fatally.

Cruz held his head in his hands for much of the playback, apparently not watching the video. Gilbert cried as she watched the video from the stand.

Several members of the public could be seen leaving the courtroom in obvious discomfort during the presentation.

Danielle Gilbert, a student at Marjory Stoneman Douglas High School during the 2018 massacre, described the helplessness and confusion after shots broke out during her AP psychology class.

"We dropped to the ground and started running towards the window, however, our teacher realized that by the window was right in view of [the door], and so she told us to come behind her desk," Gilbert testified.

"We were just kind of like sitting ducks. We had no way to protect ourselves," she said.

Gilbert said Nikolas Cruz shot into the room she was in and struck four people, one of whom later died from their injuries.

911 operators could not hear Marjory Stoneman Douglas teacher Brittney Sinitch's 911 call over the sound of gunfire, Sinitch testified Monday.

Sinitch said she called 911 after "I heard the loudest sound" of gunfire.

"It just wouldn't stop," she testified, saying the gunfire "kept going and going and going."

Prosecutors played audio from the 911 call, in which Sinitch was inaudible.

"Hello? You still there?" the 911 operator could be heard asking. "Talk to me, please!"

Brittney Sinitch, a teacher at Marjory Stoneman Douglas High School, was the prosecution's first witness to be called to the stand.

Sinitch, also an alumna of Stoneman Douglas, testified that she began her teaching career there in 2017, less than a year before the shooting.

Her voice wavered as she was sworn in Monday. Her testimony was then briefly interrupted due to technical difficulties in the courtroom.

In an Instagram post Sunday night, Sinitch said she was frightened to testify.

"I have been distracting myself all weekend and pretty much all day, but it's starting to sink in. I'm very scared," she posted.

"I'm praying for strength tomorrow. Reminding myself that I can do this -- just one step."

Jurors were given a nearly 90-minute lunch break after hearing the harrowing details of Nikolas Cruz's carnage.

The prosecution took just under an hour to deliver its opening statements Monday morning -- and will start calling its first witnesses when testimony resumes at 1:30 p.m.

Cruz's legal team had the chance to also make its opening statement Monday, but chose to wait until after the prosecution case has been presented. His lawyers are seeking to get him life in prison without the chance of parole rather than the death penalty, the only two options after he pleaded guilty in October to 17 counts of first-degree murder.

The seven-man, five-woman panel will have to agree unanimously for him to get executed, and can stop it out of mercy. However, the panelists said under oath during selection that they are capable of voting for either sentence. The trial is expected to last about four months.

With Post wires

A mother of one of Nicolas Cruz's victims left the courtroom crying as prosecutors went into gruesome detail on his systematic 2018 slaughter of 14 students and three staff.

About 50 family members of the victims were in the courtroom, some holding hands and many regularly wiping away tears as they heard the prosecution describe the deaths of their children.

It wasn't clear if anyone, aside from his defense lawyers, was there to support Cruz, who stopped scribbling and held his head in one hand as Satz described how he pulled out a vest loaded with extra ammunition and moved through the school, killing 17 and injuring others along his way.

With Post wires

Lead prosecutor Mike Satz hinged his opening arguments around the video Nikolas Cruz made three days before his 2018 slaughter.

He twice read out the confessed shooter's message, which said: "Hello, 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 going to be a big event and when you see me on the news, you’ll know who I am.

"You’re all going to die. Ah yeah, I can’t wait. Ah yeah, I can’t wait."

Nicolas Cruz fired 139 shots in his "heinous, atrocious and cold" slaughter -- hitting one of his 17 victims 13 times, his sentencing trial heard.

After the 2018 bloodbath, investigators found 70 rounds on the first floor of Marjory Stoneman Douglas High School, six in the second floor and another 61 rounds on the third floor, with two more in a stairwell where coach Aaron Feis was murdered, prosecutor Mike Satz revealed.

Of the 14 students killed, "Peter Wang was shot 13 times," he said of the 15-year-old pupil.

Satz said it was proof that Cruz, then 19, committed a "cold, calculated, manipulative and deadly" attack that "far outweigh[s] any mitigating circumstances" that Cruz's defense might offer to try to spare him from the death penalty.

He closed by again recalling a video the confessed mass shooter recorded just three days earlier saying he couldn't "wait" until everyone died.

"Those words, those actions -- of killing 14 children, the athletic director, a coach and a teacher -- is why we're here today. It's cold, calculated, manipulative and deadly," Satz said, ending his opening statements.

Nikolas Cruz casually left his slaughter to buy an ice treat -- then asked the brother of one of the girls he had just shot for a ride with his mom, the court was told.

After killing 17 inside, Cruz "runs and blends in with students and teachers who were evacuating the campus" during the fire alarm going off, lead prosecutor Mike Satz said.

He then "goes into the Subway sandwich shop" in a Walmart, where he "orders an icy" and even leaves a tip, Satz said.

Cruz then went to a McDonald's, where he sees a student who "had no idea what was going on" -- and that his sister was among the shot.

The student, who did not know Cruz, was calling his mom to pick him up because of the evacuation -- and Cruz asked him for a ride, the court heard. He was turned down, however, and arrested soon afterward.