The former head teacher of Dunblane Primary School has broken his 20-year silence to describe how he remains racked by guilt over the tragedy.
In a harrowing account Ron Taylor described how a "beautiful" spring morning in March 1996 turned into a day of "unimaginable" horror when Thomas Hamilton burst into his school, killing 16 pupils and one teacher.
"People couldn't understand why I felt so guilty after the event because there is no way any of us could have anticipated what happened," he told the Daily Mirror.
"There is no way we could have adequately prepared for what happened.
"But I felt I should have been able to do more and that guilt lives with me."
Mr Taylor described how he burst into the school's gymnasium to find dying children and Hamilton, who turned the gun on himself, twitching on the floor.
"It was unimaginably horrible to see children dying in front of you. I felt enormous guilt - more than a survivor's guilt.
"It was my school, I felt violated," he said.
The head teacher then had to undertake the grim task of helping police to identify the dead where they had fallen, before writing down the events of March 13 1996, for detectives.
To this day Mr Taylor has been unable to face re-reading the account, or look at a collection of newspaper clippings from the time.
He told the newspaper: "As a head teacher what happened to me that day was the worst experience any head teacher could have. People have to cope in their own way.
"One of the things I have at home is a box full of newspaper articles.
"And it includes my own written version of the events of the day and I did that to help. I locked it away and thankfully I have never looked at it again.
"It's quite easy to keep that box locked. It's much more difficult to keep the box in my head locked."