Three minutes and 51 seconds.

That's how long it took Nikolas Cruz to kill 17 students and staff members and injure another 17 during last year's shooting rampage at Marjory Stoneman Douglas High School in Parkland.

And that's the timeframe Pinellas County Sheriff Bob Gualtieri repeated again and again during a presentation to law enforcement officers and educators Wednesday, as he warned that more school shootings are inevitable.

“It is going to happen again. Anybody who thinks it's not going to happen again is just being unrealistic, is being naïve and probably has their head in the sand. It is going to happen again,” Gualtieri told a crowd of more than 250 people gathered at a rural counties' “Violence Against Children in a Modern Society” summit in Gadsden County.

Gualtieri, a no-nonsense law officer who is also a lawyer, has drawn national attention through his role as chairman of a statewide commission that has explored the events leading up to the horrific Broward County shooting and the response to it. The commission issued a 458-page report, which included dozens of recommendations, in January.

Much of the sheriff's three-and-a-half-hour presentation Wednesday included detailed, moment-by-moment depictions of events inside and outside of Marjory Stoneman Douglas, particulars difficult to hear for even hard-boiled detectives and deputies.

But even more painful than listening was watching a short video, compiled from footage captured by school surveillance cameras, that showed Cruz entering Building 12, unpacking an AR-15, and methodically gunning down children and teachers as he made his way from the first floor to the third floor before dropping the gun and gear beside a slaughtered student and exiting.

In one chilling scene, Chris Hixon, the school's athletic director, falls to the ground in a hallway after being shot by Cruz. An injured Hixon crawled to an alcove, where he lay bleeding, as Cruz walked by and, in Gualtieri's words, “executed him” before casually heading upstairs.

The video also showed current and former Broward County deputies, including Scot Peterson, the school's resource officer at the time, lingering outside as shots could be heard in what was known as the freshman building, taking minutes to don bullet-proof vests or hiding behind cars while Cruz carried out his deadly plan.

“Everything you're supposed to do … none of it was done,” Gualtieri said.

Peterson was arrested this month, following a Florida Department of Law Enforcement investigation into the Feb. 14, 2018, mass shooting, the worst school shooting in the state's history.

In one of his first acts after taking office in January, Gov. Ron DeSantis stripped former Broward County Sheriff Scott Israel of his post, accusing him of “incompetence” and “neglect of duty” in the Parkland mass shooting. Israel, whom DeSantis replaced with Gregory Tony, has appealed his suspension to the Florida Senate.

As Gualtieri delivered his presentation Wednesday morning, Tony held a news conference to announce that he had fired two more deputies — Edward Eason and Joshua Stambaugh — for inaction during the shooting. Peterson retired shortly after the shooting at the Parkland school, but Tony also fired him and another officer, Brian Miller, this month.

Gualtieri's presentation included video of Stambaugh taking time to put on his protective vest and waiting beside his car as gunshots could be heard. Eason also “remained stationary” outside the school, with his gun poised, but “does not move toward MSD to assist with any victims that are in close proximity to him,” despite another deputy's request for assistance, according to the results of an internal investigation released by Tony.

Gualtieri, whose commission report was highly critical of the Broward Sheriff's Office and the deputies' handling of the shooting, told reporters Wednesday that Eason and Stambaugh “failed as much as anybody else did and they should have been fired.”

Gualtieri blasted officers for failing to act when they could have saved lives.

“That's not what cops do, and they shouldn't be cops,” he said.

Speaking earlier to law enforcement officers and education workers at the summit, Gualtieri repeatedly stressed the importance of “harm mitigation.”

“Identify the threat, communicate the threat, react to the threat,” he said. “Harm mitigation is accepting the premise that it is going to happen, and you want to stop it as soon as it starts. One person shot is too many, but one person is better than 34.”

Gualtieri showed little patience for schools that are not complying with safety requirements passed by the Legislature last year after the shooting and strengthened during this year's legislative session.

“Focus on the low-cost and no-cost things like having a hard-corner policy and having training,” he said. “There are so many things that can be done to make a difference and make the schools safer, that don't cost anything. There is no excuse for not doing it. All it takes is a decision maker to have the will to say, 'We are doing it' and to hold people accountable for not doing it.”

For example, schools need to conduct active shooter drills to prepare students and faculty, he said, pointing out that Peterson underwent a single, one-hour active shooter exercise in the three years leading up to the mass shooting.

Some school leaders have objected to the drills, Gualtieri said.

“So guess what, they're not doing it,” he said, adding that others are “playing games with it.”

“They're having drills by doing what we're doing here. Sitting talking about it. … That's a bunch of nonsense,” he said. “They don't like it. Guess what. I don't like dead kids. … Go take a look inside that school and see those kids' blood on the floor. … Then see what you want to do.”

Two Parkland students died because they were unable to join their classmates in a “hard” corner, Gualtieri said. A safe corner in another classroom was blocked by a desk, he said, blaming the Broward school system for failing to ensure such a system was in place.

“This is a hard thing to say but because of their lack of a policy and educating the teachers and making sure that the safest area in the room was available, kids died because of that,” he said.

This year's school-safety legislation expanded the “guardian” program, which authorizes specially trained school personnel to be armed. The expansion will allow full-time teachers to join the program, a move that was recommended by the commission.

Gualtieri at one time had insisted that only trained law-enforcement officers should be allowed to carry guns at schools, but he said his view has changed, “based on facts, based on evidence, and based on what we have learned.”

Only half of Florida's 4,000 schools have a full-time school resource officer, who is usually a deputy, Gualtieri said. And police departments are unable to fill vacancies. He said expanding the pool of people — “with the right mindset, the right skill set” and rigorous training — who can bring guns to schools will make campuses safer.

Opponents of allowing teachers to participate in the guardian program served up a “parade of horribles” as the proposal was being considered this year, Gualtieri said.

“What if this. What if that. I don't know. What if? What if we got 34 shot and killed people in three minutes and 51 seconds? No greater parade of horribles,” he said.

Dara Kam reports for the News Service of Florida.