(CNN) When Scot Peterson arrived at the 1200 Building, the scene of the Parkland, Florida, school massacre, it was pandemonium and he didn't have time to be scared "because I was doing things the whole time," he said in an interview that aired Tuesday.

Dubbed a coward for his response to the Marjory Stoneman Douglas High School shooting that killed 17 people, the school resource officer has been making the media rounds to tell his side of the story.

A reporter for The Washington Post spent time with the former Broward County deputy for a feature that ran Monday , and NBC's "Today" show ran the first of a two-part interview Tuesday.

Speaking to "Today," Peterson conceded he "didn't get it right" when he arrived on the scene of the shooting, but it wasn't because he was scared or reluctant or because he froze up. Rather, in the chaos of the shooting, he had "no real-time intelligence whatsoever," he said, referencing reports that 911 calls were going to Coral Springs, just south of Parkland, rather than to Broward County.

In the short amount of time he had to assess the situation, he couldn't tell at first if someone had ignited firecrackers, if there was a sniper on campus or if shots were coming from inside or outside the building, Peterson told the morning show.

"Those were my kids in there. I never would've sat there and let my kids get slaughtered," he said.

Once he realized there was a shooter, he followed his training, taking up a position where he could see three buildings and scan for a sniper, he told "Today." But because the three-story 1200 Building has hurricane glass, he couldn't determine the origin of the shots, he said.

Scot Peterson in his "Today" interview.

Asked pointedly about video that appears to show him standing around outside as the shooting continues, as well as about dispatcher audio in which he mentioned the 1200 Building, Peterson said, "I didn't believe there were students anywhere mingling around, walking around. ... I never believed there was an active shooter inside."

In hindsight, Peterson said it's easy to say what he should have done. But that wasn't the case amid the chaos, he said.

"Knowing what I know, it's awful. Like I said, it haunts me," he said. "I''m never going to get over this."

'I lost 17'

Being haunted was a theme of his interview with the Post. He told the paper he knows he could have done more that day and struggles with a torrent of what-ifs.

Though the reporters are no longer camped out in the cul-de-sac of his retirement community, Peterson still has a sheet draped over his front door to fend off any prying eyes that might be outside his home, the Post reported. His girlfriend, Lydia Rodriguez, plays Christian music and posts affirmations -- such as "Rule your mind, or it will rule you" and "Cherish Yourself!" -- around the house to help Peterson get out of his own head.

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Neighbors and friends bring over cookies and meet him for lunch in an effort to lift his spirits after he was pilloried by his former boss, the parents of slain children and President Donald Trump

"It's haunting," Peterson told a neighbor, identified by the Post only as Jim. "I've cut that day up a thousand ways with a million different what-if scenarios, but the bottom line is I was there to protect, and I lost 17."

"Come on, now," Jim replied. "It's not all on you."

"But that's the perception," Peterson said, according to the paper. "You're a hero or a coward, and that's it."

'The evidence is sitting right there'

At his duplex in Boynton Beach, Peterson studies documents and surveillance footage from the shooting and walks through his own actions, pondering what he could have done differently. He vacillates between disgust at those who said he let kids die and anguish over knowing that as the only armed law enforcement officer on campus, he was best positioned to take down the shooter.

Surveillance video showing Peterson with his back against a wall outside, never entering the building, is fodder for those who accuse him of doing nothing. But he told The Post he was in reality calling in the shooting on the radio, locking down the school and clearing kids out of the courtyard.

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"How can they keep saying I did nothing?" he asked Rodriguez, according to the newspaper. "I'm getting on the radio to call in the shooting. I'm locking down the school. I'm clearing kids out of the courtyard. They have the video and the call logs. The evidence is sitting right there."

Added Rodriguez, "It's easy to second-guess when you're in some conference room, spending months thinking about what you would have done."

Peterson loved the children of Marjory Stoneman Douglas High School, and it pains him to think they would view him so dimly after all the years he spent protecting the campus, the Post reported.

There was no time to think, he told the paper. He was in his office, a few hundred yards away from the 1200 Building, when he got the call on Valentine's Day. At first he believed someone had ignited firecrackers on campus. Soon, it became clear the pops were gunshots, but their origin remained uncertain. One call suggested they might be coming from the football field

'Stop torturing yourself'

Peterson had attended conferences and training sessions for active shooter incidents. He had envisioned his response: finding, engaging and killing the shooter.

But when shots rang out, he found himself outside frantically trying to determine where the shooter might be. According to the paper, he stayed outside because he didn't want to expose himself when he wasn't sure where the gunman was. His first instinct when he got home that day was that he messed up -- and that was long before he was dubbed the "coward of Broward."

"I couldn't get him," he remembered telling Rodriguez after the incident. "It was my job, and I didn't find him."

He's seen a psychologist and psychiatrist. Prescription medication helps him sleep. Professionals say he's suffering from a variety of symptoms, including confusion, anxiety, guilt, grief, agitation and obsession.

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The obsession was evident as he watched a 3-D re-creation of the shooting, much to Rodriguez's chagrin. In it, he retraced the steps of the shooter, represented by a black rifle, in an effort to reconcile them with his own.

"I was trying to figure it out," Peterson said, according to the Post. "I was scanning for the shooter, looking over the windows, the sidewalk, the rooftop. I thought maybe it was a sniper like in Las Vegas. I just didn't know."

Later, he traced his finger over the stairs, saying, "I could have come in over here. ... I could have got him while he was reloading. If I'd just heard more shots, maybe I would have known where they were coming from."

Rodriguez, worried that reliving the incident is unhealthy for her beau, told him, "Stop torturing yourself."

As described by the Post, Peterson didn't stop. He watched until the black rifle left the school.

"It's brutal," he said.