Deputy Scot Peterson has resigned after being suspended, Broward County Sheriff Scott Israel said.
“I am devastated. Sick to my stomach. He never went in,” Sheriff Israel said.
Earlier this week President Trump said arming school teachers could prevent school shootings.
The proposal has long been championed by the powerful National Rifle Association (NRA) gun lobby.
On Thursday, in the NRA’s first comments since the massacre, its head accused Democrats and media of “exploiting” the attack. Wayne LaPierre said “opportunists” were using the tragedy to expand gun control and abolish US gun rights.
What did the officer do?
Sheriff Israel said Mr Peterson, 54, was the school resource officer of the Marjory Stoneman Douglas High School, and was at the site, armed and in uniform, when the shooting took place.
He said video footage showed Mr Peterson arriving at the building where the shooting broke out about 90 seconds after the first shots were fired, and that he remained outside for about four minutes. The attack lasted six minutes, Sheriff Israel said.Asked what Mr Peterson should have done, Sheriff Israel said: “Went in, addressed the killer, killed the killer.”
Mr Peterson is yet to publicly comment on what happened. Officers are reportedly guarding his home.
Sheriff Israel said Mr Peterson had not given a reason for why he did not go into the building where the shooter was. It is unclear if he will face charges.
Sheriff Israel said he would not be releasing the video showing Mr Peterson and might never do so, “depending on the prosecution and criminal case” against the 19-year-old suspect accused of carrying out the shooting, former student Nikolas Cruz.
The gunman used a semi-automatic AR-15 assault rifle, police say, and escaped the scene before being seen and arrested later.