The former school resource officer at Marjory Stoneman Douglas High School, who did not enter the building where a mass shooter killed 14 students and three adults at the South Florida school on Feb. 14, expressed regret in his first public interview that he did not intervene.

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"Knowing what I know today, I would have been in that building in a heartbeat," Scot Peterson told NBC News' Savannah Guthrie on Today, in an interview that will air Tuesday and Wednesday on the morning program.

The interview will be aired two  days after graduation ceremonies were held at Stoneman Douglas. Four of the students killed were seniors.

Guthrie asked Peterson if he could acknowledge that “You missed it,” referring to his response when Nikolas Cruz entered the building in the South Florida high school and opened fire.

“I have to,” Peterson said. “I have to live with that. You know, how could I not?

“I mean, I'm human … in the perfect world, oh, I would have said, ‘Oh, yeah, I know there was a shooter in there. Let me go to the third floor. Find this person.’”

Peterson, 54, began working at the Parkland high school in 2010. He spent nearly three decades with the Broward County Sheriff’s Office.

After the incident, Broward County Sheriff Scott Israel criticized Peterson’s lack of action and suspended the deputy without pay. Peterson then decided to retire.

His actions were criticized by students and parents, and President Donald Trump called Peterson “a coward.”

The father of Meadow Pollack, who was killed in the Valentine’s Day shooting, filed a wrongful death lawsuit in Broward County that included Peterson as a defendant.