When the 1st shot rang out, other than the driver Bill Greer & front seat passenger, Roy Kellerman, all other Secret Service agents were riding on or in the "follow-up" car.
The speculation that JFK ordered agents not to ride on his car during the motorcade is discussed in the new book "The Kennedy Detail" by Gerald Blaine.
Clint Hill, on the left front side running board of the follow up car, sees JFK slump & his hands come up toward his throat. He leaps off the running board & races to the Presidential car.
"Clint Hill was just feet from his goal, his eyes focused on Mrs. Kennedy, when he heard the third shot and the gruesome thump of President Kennedy's head exploding."*
Mrs. Kennedy screams out:
"They've killed my husband."
She climbs out on the trunk as Clint Hill reaches the rear step & hand hold.
****Hill said later she was reaching for (a piece of skull) on the trunk.
Elm Street & Dealey Plaza
Dallas, Texas (2008)
Photo by Lowesvisa at en.wikipedia