After three months of silence, the Florida sheriff's deputy forced to retire after he failed to confront the Parkland school massacre gunman has something to tell the families of the 17 victims.

"I'm sorry," Scot Peterson said through tears in an interview with "Today."

The 33-year law enforcement veteran said it wasn't fear that kept him from rushing into Marjory Stoneman Douglas High School as Nikolas Cruz stalked the halls with an AR-15. It was chaos, miscommunication and his assumption that the shots were being fired outside by a sniper.

"I didn't get it right," Peterson admitted. "But it wasn't because of some, 'Oh, I don't want to go into that building. Oh, I don't want to face somebody in there.' It wasn't like that at all."

Watch the second half of the interview with Scot Peterson on “Today” Wednesday morning.

"Those are my kids in there," he added. "I never would have sat there and let my kids get slaughtered. Never."

Peterson, a Broward County school resource officer, was the only other person with a gun on campus when Cruz, 19, opened fire inside Building 1200 at 2:21 p.m. on Valentine's Day.

After security video showed Peterson never tried to enter the building, taking cover against a nearby wall instead, he was branded the "Coward of Broward."

President Donald Trump denounced Peterson, saying "he didn't have the courage." Broward County Sheriff Scott Israel said the deputy's actions made him "sick to my stomach." The father of a slain teen filed a wrongful death suit against him.

I filed a wrongful death suit against Deputy Peterson today. I want to expose that coward so bad. Where ever he goes I want people to recognize him and say that's one of the cowards of Broward. The SRO that let those children and teachers die on the 3rd floor! — Andrew Pollack (@AndrewPollackFL) April 30, 2018

Through it all, Peterson stayed quiet and out of the spotlight. But now, he said, "enough's enough."

He said he is giving his side of the story for "the families." "They need to know the truth of what I did and not that false narrative that Sheriff Scott Israel gave," he said.

Although he has been accused of doing nothing to stop the killer, Peterson said that he jumped into action the minute he got a call of firecrackers going off and never stopped.

He ran from his office and went with a security specialist to get a golf cart to cross the 45-acre campus. They didn't have the keys so he headed on foot to Building 1200. As fire alarms began going off, a security monitor, Drew Medina, picked him up in a cart and drove him toward the building.

"As we approach near the building I hear two to three loud shots. I immediately stop and I'm thinking to myself, 'Oh my God, I hear shots outside,'" he said.

"I went right to my [police] radio. I screamed, 'Shots fired.' I then went to the school radio and I said, 'Put the schools in lockdown.' I'm yelling at the security specialist, 'Get out of here. Get out of here.' Because I thought the shots were outside and I didn't want him standing out there.

"And immediately I went and took a position of cover over by the 700 building, because that's what we're trained [to do]."

With his gun out, he said, he began looking at the roof and windows of 1200, hoping to spot the shooter. Over his radio, he ordered responding officers to stay away because he was worried they could be hit by sniper fire.

Peterson said he never thought the gunman at Marjory Stoneman Douglas was targeting students and staff inside — even though school shootings have become tragically common, even though he had trained school staffers on how to respond to an active shooter, and even though he said over the radio at one point that he heard shots "by, inside the 1200 building."

"It haunts me that I didn't know," he said. "I was trying to do the best I could with no information or intel at the time. ... And it was just something happening so fast."

In fact, by the time Peterson got to Building 700, Cruz had already killed 11 people on the first floor. Cruz then went to the third floor and continued to shoot for two minutes, killing six more as the deputy stayed pinned against the wall.

Peterson said he never heard any more shots that would have alerted him to the suspect's location. He wondered if the thick windows on the hurricane-proof building might have muffled the sound.

He also said he didn't hear police radio transmissions indicating Cruz was in the building because cellphone calls to 911 were relayed to Coral Springs dispatchers, not to Broward County dispatchers. One arriving officer even indicated there was shooting on the football field, he said.

He also pointed fingers at Drew Medina, the school monitor who picked him up in the golf cart. He said he later learned that Medina had seen Cruz get out of an Uber and hurry toward Building 1200 with a bag just before the shooting.

Medina gave investigators a similar account, saying he even warned another school monitor of a suspicious person headed for Building 1200, according to a sworn statement obtained by The Fort Lauderdale Sun-Sentinel. But Medina now denies the details in the sworn statement, according to the newspaper.

Peterson's attorney, Joe DiRuzzo, said the Broward County Sheriff's Office had made his client "a scapegoat" to deflect criticism of its own actions. Peterson, who has four grown children, now lives as a semi-recluse with his girlfriend in a Boynton Beach duplex, a sheet over the front door, according to The Washington Post.

He cringes when his name is mentioned on the news or in headlines, and he obsessively replays the events of Feb. 14 in his head, the newspaper said. He worries when he shows his identification somewhere that he'll be recognized.

"It's been hard," he told NBC News. "But it's nothing compared to what these families went through. They lost their kids."

He said he wishes he could speak to the families and explain what was going through his mind as he stayed pressed up against the wall during Cruz's rampage.

What would he say?

"I'm sorry," he said through tears. " I'm sorry that I didn't ... I'm sorry that I didn't know where he was."