But he knew guns and he knew better. Then came another shot. And another. And the president slumped down.

For so many nights afterward, he relived that grisly moment in his dreams. Now, 60 years later, Paul Landis, one of the Secret Service agents just feet away from President John F. Kennedy on that fateful day in Dallas, is telling his story in full for the first time. And in at least one key respect, his account differs from the official version in a way that may change the understanding of what happened in Dealey Plaza.

Mr. Landis has spent most of the intervening years fleeing history, trying to forget that unforgettable moment etched in the consciousness of a grieving nation. The memory of the explosion of violence and the desperate race to the hospital and the devastating flight home and the wrenching funeral with John Jr. saluting his fallen father — it was all too much, too torturous, so much so that Mr. Landis left the service and Washington behind.