The armed officer – branded a "coward" by President Donald Trump after he failed to confront the Florida high school gunman – has defended his actions during the mass shooting on Valentine's Day.
Scot Peterson's lawyer Joseph DiRuzzo insisted the officer did not enter the building at Marjory Stoneman Douglas High School as he believed the gunfire was coming from outside.
The reaction comes after Mr Trump criticised officers who were there, saying they "weren't exactly Medal of Honour winners".
The US leader also said he would have gone into the school himself during the attack even if he was unarmed.
In a statement, Mr DiRuzzo said it was "patently untrue" that Mr Peterson failed to meet standards or that he acted with cowardice during the attack, which saw 17 people shot dead at the school.
"Let there be no mistake, Mr Peterson wishes that he could have prevented the untimely passing of the 17 victims on that day, and his heart goes out to the families of the victims in their time of need," he added.
The statement follows Mr Peterson resigning from his post after Broward County Sheriff Scott Israel criticised him for failing to intervene.
Mr DiRuzzo said the sheriff's account of Mr Peterson's actions that day was a "gross oversimplification".
The sheriff's office declined news agency requests for comment but said Mr Peterson's conduct was under investigation.
Mr Peterson and a security officer ran to the scene when they were alerted to the shooting – which was reported as firecrackers being set off near a building, according to the officer's statement.
He then heard gunshots "but believed that those gunshots were originating from outside of the buildings".
The officer "took up a tactical position" between two nearby buildings while alerting dispatchers and initiating a "code red" lockdown.
The statement said "radio transmissions indicated that there was a gunshot victim in the area of the football field" adding to Mr Peterson's belief the shooting was taking place outside.
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It follows Mr Trump's comments to governors at the White House. He said: "You don't know until you're tested, but I think I really believe I'd run in there even if I didn't have a weapon.
"And I think most of the people in this room would have done that, too."