3.The USSS agents started protecting the President 24/7 after the assassination of President William McKinley in 1901.
When the presidential candidate Robert F. Kennedy was assassinated, the USSS agents started protecting the presidential and vice-presidential candidates too in 1968.
4.So far only one USSS agent has died on the job.
In 1950 when President of that time Harry Truman was staying somewhere else. Two Puerto Ricans entered the White House to assassinate the President.