Attempted assassination of Harry S. Truman
On November 1, 1950, Puerto Rican secessionists Oscar Collazo and Griselio Torresola attempted to assassinate President Harry S. Truman at Blair House during the renovation of the White House. Both men were stopped before gaining entry to the house.
Source: Wikipedia — Attempted assassination of Harry S. Truman (CC BY-SA 4.0)