Only after attacks on the president happened did the agents' duties expand.
Following the 1901 assassination of President William McKinley, Secret Service agents began watching over presidents full time. After Sirhan Sirhan shot and killed presidential candidate Robert F. Kennedy in 1968, agents took on the job of protecting presidential and vice-presidential candidates as well. In fact, the Service offered Barack Obama protection a year and a half before election day, the earliest of any candidate in history.
Only one member of the Secret Service has died protecting the President.
On November 1st, 1950, two Puerto Rican nationalists stormed the house that President Harry Truman was living in while the White House underwent renovations. In their attempt to assassinate Truman, they shot Private Leslie Coffelt, a member of the Secret Service, three times. Before he died from his injuries, Coffelt returned fire, shooting one would-be assassin in the head.
The United States Secret Service uses code names for U.S. presidents, first ladies, and other prominent people and places. John and Jackie Kennedy were known as "Lancer" and "Lace"; Ronald and Nancy Reagan went by "Rawhide" and "Rainbow." Agents call the Pentagon "Calico," and refer to The White House as "Castle."
In 1908, the Department of Justice hired Secret Service agents to conduct national investigations. The nine agents formed the Bureau of Investigation, later renamed the Federal Bureau of Investigation.