His attorney, Joseph DiRuzzo, said his client believed the gunfire was coming from outside the school.
He followed his training by taking cover and prompting a lockdown, the lawyer said.
Mr DiRuzzo said it was "patently untrue" that Mr Peterson was a coward on the day.
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Mr Peterson said he had originally "received a call of firecrackers".
Only when he ran towards the sound, he said, did he realise it was gunshots.
Broward Sheriff's Office had trained him to seek cover and assess the situation in the event of outdoor gunfire, his lawyer said. Mr Peterson did so, and then told the sheriff's office he had heard shots, prompting a "Code Red" lockdown of the school campus.
When police arrived, he told them he thought the gunman was outside - a belief backed up by "radio transmissions [which] indicated that there was a gunshot victim in the area of the football field," Mr Peterson said, according to his lawyer.
His attorney said in a statement: "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.
"However, the allegations that Mr Peterson was a coward and that his performance, under the circumstances, failed to meet the standards of police officers are patently untrue."