After JFK: How the Secret Service Has Changed

Kennedy's assassination in 1963 served as the crucible for presidential protection.

The limousine carrying mortally wounded President John F. Kennedy races toward the hospital as Secret service agent Clinton Hill jumps in. (Justin Newman/AP)

Corrected on : Corrected, 11/19/13: An earlier version of this article misidentified Jeffrey Engel, a professor at Southern Methodist University.