The chief responsibility of the U.S. Secret Service is to guard the life of the president.
In Dallas, on Nov. 22, a sniper hidden in an office building shot and killed President John F. Kennedy, ending a spotless record by the Service.
[READ MORE: JFK: 50 Years Later]
It was the first time since the Secret Service took over its protective mission 62 years ago that a president had been harmed.
Guarding the president has become more and more of a problem in recent years. Presidents travel a lot more -- at home and abroad -- and are frequently exposed to big crowds.
Widespread safeguards are taken to protect the life of a president. He and his family are accompanied by Secret Service men wherever they go.
Crank letters are carefully screened, and files are kept of every threat.
When a president travels, his route is carefully checked in advance. Elevator cables are tested. Fire precautions are doubled. Everyone dealing with his meals undergoes a security check.
Every time he uses a helicopter, two agents -- each trained to land the vehicle safely -- travel with him.
The White House has a limousine fitted with a removable "bubble top" of transparent material. In practice the top has been used mainly as protection against rain. The top was not in place when President Kennedy was shot while riding in the limousine.
Every president is under constant pressure to follow the security advice of the Secret Service. U.S. law "authorizes" the Secret Service to protect the president. It does not spell out authority for the Service to control the president's movements. Agents accompany the president whenever he is outside his private living quarters, are nearby at all times. But it is the president who makes the decisions.
President Kennedy early gave the Secret Service notice of what to expect. At his inaugural ball, he "table hopped" to chat. On short notice he visited a movie theater.
On a California visit, he slipped out to swim in the surf. At Cape Cod, he often left his yacht for a swim.
President Kennedy also upset Secret Service men by driving his own car whenever he got the chance. He would often approach a waiting crowd and shake as many hands as he could while his protectors worried.
On his most recent trip to New York the late president irked both the Secret Service and New York police by having his chauffeur observe all traffic regulations, including stop lights. Passers-by rushed up to surround his car, and caused traffic jams.
In his actions, Mr. Kennedy apparently was unmindful of the advice former Vice President Garner gave Mr. Truman when the latter inherited the presidency: "Say your prayers and mind the Secret Service."
This article originally appeared in the Dec. 2, 1963, issue of U.S. News & World Report. For more about John F. Kennedy, visit JFK: 50 Years Later.