The surveillance video shows Scot Peterson, whom President Donald Trump dubbed a "coward" in the attack's aftermath, arriving at Parkland's Marjory Stoneman Douglas High School but standing outside rather than entering.
The images do not show the carnage that Nikolas Cruz, 19, carried out with an assault rifle.
But they do expose the 54-year-old Peterson's failure to respond, authorities say.
"The video speaks for itself," the Broward County Sheriff's Office said.
Feb 14 2018: Deputy Scot Peterson, right, standing outside the Marjory Stoneman Douglas High School. Source: AAP/Broward County Sheriff's Office
"We welcomed the court's decision to release the video of Deputy Scot Peterson's actions on February 14," the office's statement said, saying the ex-officer's "actions were enough to warrant an internal affairs investigation."
"After being suspended without pay, Peterson chose to resign and immediately retired rather than face possible termination."
The footage shows a man, identified by police as Peterson, at 2:23 p.m. approaching the wall of the school's Building 12 and standing there for several minutes.
On February 22 Commissioner Scott Israel disclosed that Peterson, who was assigned to monitor the campus, had not entered the building to try and thwart the massacre.
Israel made clear he believed the deputy's actions were negligent, suspending him without pay. Shortly thereafter Peterson resigned.
Following Israel's comments, President Donald Trump attacked the deputy by name the next day, saying he either froze or was a "coward."
Feb 14 2018: Deputy Scot Peterson, right, leaving the Marjory Stoneman Douglas High School after standing at the front door. Source: Reuters/CCTV image grab
Peterson broke his silence via his lawyer nearly two weeks after the shooting.
"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," his attorney, Joseph DiRuzzo III, said in a statement.
"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," DiRuzzo added.
Cruz was charged with 17 counts of first-degree murder and a trial date has yet to be scheduled.
The US state of Florida intends to seek the death penalty. A judge on Wednesday entered a plea of not guilty on Cruz's behalf.
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