Agent Hill—poised just a few feet behind the Kennedys, who sat in the backseat of the car in front of him—was nervous, according to accounts he has written about the events in Dallas in his books. The motorcade kept speeding up and slowing down, speeding up and slowing down. That morning, he frequently jumped off the running board to jog alongside the vehicle. Kinney, right behind him in Halfback’s driver’s seat, watched him struggle to keep pace with the cars.

Hill, because he had been assigned to the First Lady’s detail, had the job of focusing on her much more than on the president. By his own admission, Hill maintained a close relationship with Mrs. Kennedy. They had become friends, he noted, after a propitious encounter. One day when he was seated next to the driver as the First Lady was being chauffeured outside Washington, he lit up a cigarette. She requested that the driver pull the car over. Hill, expecting to be reprimanded, was surprised when she asked him to join her in the backseat, then bummed a smoke. The driver continued on and the two sat puffing and chatting away.

When the first shot rang out, Hill recalls now, “I described it as an explosive device. It resembled a firecracker, but a loud one, and it came over my right shoulder from the rear. I wasn’t absolutely sure what it was. I turned toward the noise.” Hill scrambled to reach the presidential limousine. He says it took six seconds to get there, and the home-movie footage of the assassination shows that he was the only Secret Service agent within reach of the car to have moved toward the back of the limo. Indeed, he climbed onto the Kennedys’ car and pushed the First Lady—who had crawled up onto the trunk—back into her seat. By then, it was too late to save the president.

Across from Hill, Agent Jack Ready was also on the footboards, yards from the president. Behind Ready, in similar proximity, was Paul Landis, also from Mrs. Kennedy’s detail. “I knew right away it was a gunshot,” Landis, now 79, tells Vanity Fair, from his home near Cleveland, Ohio. “I was a hunter. I’ve done a lot of shooting. There was no doubt in my mind, in fact.” (Ready, according to Landis, guessed that it was a firecracker.) But in the blur of the action, Landis, although noticing the president “leaning to the left,” says he did not connect the sound of the gun and J.F.K.’s movements.

As it turned out, three of the men just behind the limo, including Landis and Hill—along with Glen Bennett, who was inside their chase car—had gone to the Press Club and then the Cellar the night before. Hill, Ready, and Bennett, by their own statements to the Warren Commission, had stayed until close to three A.M.; Landis, for two hours longer. (Ready died in 2014. At press time, vanityFair.com was unable to locate Bennett.)

Landis, when asked what, if anything, might have impeded their actions that day, says that the question of “the drinking was blown out of proportion . . . So other than that, I think you could say, ‘lack of sleep.’ But you’re wide awake . . . going on adrenalin.” Did his being out all night factor into it? “That’s a tough one,” he says. “I don’t think that affected me. That’s an arguable point.”

By way of contrast, the fourth car in the motorcade that day, containing Vice President Johnson and his wife, was guarded by other agents, including Rufus Youngblood. Youngblood had not joined the others at the Press Club and the Cellar the previous evening. And at the sound of the first shot, Youngblood, in line with his Secret Service training, pushed Johnson to the floor of the car and covered him with his own body.

A week after the events in Dallas, President Lyndon Johnson ordered the first official investigation of the assassination. Its chairman: Earl Warren, the chief justice of the Supreme Court, who had had a distinguished career as the governor of California. The Warren court, during his 15 years presiding, would hear many pivotal cases, including the landmark civil-rights decision affirming Brown v. Board of Education, in 1954. And with his reputation established and mindful of the political thicket that would surround an investigation into the assassination, Warren had to be bullied by the new president into accepting the post.