"It was like meeting a movie star," said Hargis. "He said, 'I'm glad you're here. Thank you for being here.' "

On Cedar Springs, the president startled Hargis by leaping out of the car to shake hands with some of the hundreds who pushed forward for a closer look.

"The Secret Service liked to had a conniption fit when he did that," says Hargis. At that moment, he felt an eerie sense of dread wash over him. "They was hoppin' around like cats on a hot roof. It freaked 'em out big time. You could tell how nervous they were."

But once the procession reached Houston Street, preparing for the final turn onto Elm, Hargis began to relax. "I thought, 'Well, we've got it made now,' " he says. "And then bam! It happens."

Hargis differs with the Warren Commission and most eyewitnesses, insisting that only two shots were fired. With the first, "a thousand million things went through my mind," he says. After the last, "there was a plume of blood and brains and plasma. It was just like a fog, and I ran right through it."

Photographs taken in the seconds that followed show Hargis racing up the grassy knoll in pursuit of a sniper. He thought the shots came from there. But like most motorcade survivors, he believes Oswald acted alone, saying he saw "nothing" behind the knoll's picket fence to indicate anything suspicious — much less a second gunman.

"When I reached the School Book Depository, someone said, 'Bob, you've got something on your lip.' It turned out to be a piece of brain, mixed with bone from the president's skull."

Within seconds, a man approached Hargis, vowing, in the officer's words, "to get his hands on $17,000 if I'd agree to sell him my helmet. I couldn't sell it anyway. It belonged to the city of Dallas."

After the assassination, "the whole country changed," he says. Before then, "everything was so naive. ... We believed that everything was going to be fine, even if things didn't go right. But now, you can't believe that."

Two years after the president died, Hargis suffered his own near-fatal injury while patrolling on a police motorcycle. The accident crushed his leg and shattered his ribs. He took medical leave from 1974 to 1980, when he returned to the force. He held an administrative job until 1999, when he retired.

"The assassination made me more cautious and careful about every aspect of life," he says. And it continues to haunt him even now, even when he's asleep.

In dreams, he still chases the killer, spending what seems like hours racing up and down the stairways of the School Book Depository, almost touching the fabric of Oswald's shirt but never quite pulling him in.

"It starts out a normal dream and ends up a nightmare. Every single time," he says.