Veteran 60 Minutes correspondent Mike Wallace once said that, in all his years as a journalist, very few interviews stayed with him like his time with Clint Hill, a former U.S. Secret Service agent on duty the day President John F. Kennedy was assassinated. During that interview, Hill stunned Wallace -- and the nation -- by admitting he felt responsible for the president's death. Hill would later say it was the first time he had ever spoken publicly about that day, and that his emotional reaction surprised even him. Now at 92 years old, Hill tells 60 Minutes: A Second Look why he spoke so candidly for an audience of millions, and how that interview with Mike Wallace may have changed the course of his life.