NATIONWIDE -- The deputy maligned by many for not rushing into Marjory Stoneman Douglas High School during the Valentine's Day massacre at the school spoke out Tuesday.

• Ex-deputy Scot Peterson speaks out about Parkland shooting
• Peterson widely criticized for response during massacre
• Peterson said Florida school shooting still haunts him

"… I believed there was a sniper, so in my mind, I'm thinking to myself, 'There's possibly someone up in there shooting out,' " Scot Peterson told Savannah Guthrie on NBC's Today Show, trying to explain why he took up a position outside a school building. "But I didn't think they were shooting at the kids. I thought they were shooting out at the building," he said.

Peterson has been widely criticized since the Feb. 14 massacre that left 17 people dead at the high school in Parkland, Fla.

He resigned and retired from the Broward County Sheriff's Office eight days after the shooting, when video surveillance footage showed him standing outside the building during the rampage.

Peterson was in his office in another building when he got reports of sounds of "firecrackers" on campus.

He arrived at the building at about 2:24 p.m., just as authorities say the suspected gunman was heading to the third floor. Six people were killed on the third floor, 11 on the first floor.

Peterson said he was getting "no real-time intelligence whatsoever." He said there were conflicting reports on his radio, and one report even had the shooter at the football field.

The former deputy said he wants the families of those killed to know that "I didn't get it right." But he said it wasn't because he was afraid to enter the building.

"If I'm a parent whose child died in that building, you do think there's a shooter inside -- why aren't you going inside?" Guthrie asked him during the interview.

"Because, Savannah, I didn't know if (the shooter) was in or outside," he replied.

"Why not check it out?" she asked.

He then told Guthrie that he was trained to "contain the area."

Peterson said the mass shooting haunts him.

"It's easy for people to sit there and say, 'He should have known that person was up there.' It wasn't easy," he said. "I'm never going to get over this."

The Associated Press contributed to this story.