Florida authorities on Thursday released security footage appearing to confirm that an ex-deputy failed to take action during last month's school shooting massacre that left 17 people dead.

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.

"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, Sheriff 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, Trump attacked the deputy by name the next day, saying he either froze or was a "coward."

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.

The 19-year-old Cruz was indicted last week on 17 counts of first degree premeditated murder and 17 counts of first degree attempted murder. The attempted murder charges relate to those wounded in the attack.

The state of Florida said on Tuesday it intends to seek the death penalty for Cruz. A judge on Wednesday entered a plea of not guilty on Cruz's behalf.

The FBI admitted after the shooting it had received a tip that Cruz had a "desire to kill" and access to guns and could be plotting an attack, but agents failed to investigate.

A person who was close to Cruz had called the FBI's tip line on January 5 and provided information about Cruz's weapons and his erratic behavior.

The FBI acknowledged that the tip should have been shared with the FBI's Miami office and investigated, but it was not.

AFP contributed to this report.