They didn't fire any shots that day, but the Secret Service killed him through incompetence.
"After the shooting, one of the secret service men sitting down in the car in front of us pulled out an automatic rifle or weapon and looked backward. However, all of the secret service men seemed to me to respond very slowly, with no more than a puzzled look. In fact, until the automatic weapon was uncovered, I had been lulled into a sense of false hope for the President's safety, by the lack of motion, excitement, or apparent visible knowledge by the secret service men, that anything so dreadful was happening. Knowing something of the training that combat infantrymen and Marines receive, I am amazed at the lack of instantaneous response by the Secret Service, when the rifle fire began. I make this statement in this paragraph reluctantly, not to add to the anguish of anyone, but it is my firm opinion, and I write it out in the hope that it might be of service in the better protection of our Presidents in the future" Ralph W. Yarborough