An armed officer failed to confront the Florida school shooter despite being at the scene, officials have claimed.
17 students and staff died in the shooting at Marjory Stoneman Douglas High School.
The local sheriff's office says Deputy Scot Peterson was on duty when the shooting started, but never entered the building.
The deputy - who was working as a school resource officer - has resigned from the force, after initially being suspended pending an internal investigation.
Broward County Sheriff Scott Israel says the video evidence from the day of the shooting is damning.
He said: "What I saw was a deputy arrive at the west side of building 12, take up a position... and he never went in."
Giving his response to the latest developments, Sheriff Israel added that he was "devastated. Sick to my stomach. There are no words."
19-year-old shooting suspect Nikolas Cruz has been charged with 17 counts of premeditated murder following last week's attack.
Staff are today returning to the school in Parkland, for the first time since the shooting.