In 1950, two Puerto Rican nationalists attempted to assassinate President Truman during his stay at the Blair House, while the White House was being renovated. They began by ambushing Leslie Coffelt, who was a member of the White House Police, which would become part of the Secret Service in 1977. Coffelt, standing guard, was shot multiple times in the chest by would-be assassin Griselio Torresola, who then shot other members of the guard force before moving to enter the Blair House. Coffelt was still conscious, however, and rose to fatally shoot Torresola in the back of the head from the guard booth. Coffelt then collapsed and died of his wounds. Coffelt was buried in Arlington Cemetery.
Oscar Collazo, Torresola's accomplice, was shot by other officers but survived. In 1979, President Jimmy Carter commuted his sentence and he returned to Puerto Rico.
Newsreel about the attempt (Warning contains NSFW gore/images of dead bodies):
https://www.youtube.com/watch?v=8wGYoU50uks
Sources:
Truman Library website:
https://www.trumanlibrary.org/trivia/assassin.htm
Article concerning the 40th anniversary of the attempt: