The USA has lost four presidents to assassination; they are
1. Abraham Lincoln (1865)
2. James Abram Garfield (1881)
3. William McKinley (1901)
4. John F Kennedy (1963)
All four were shot; the first three at close range. Two were shot in the head (Lincoln and Kennedy) and at least one was killed by a foreigner.
Interestingly, at least one person was either present or close in distance to the scene of assassination for the first three slain presidents. Robert Todd Lincoln, a lawyer and the oldest surviving son of Abraham Lincoln, knew each president well and just happened to be near the crime scene for the deaths of the three presidents killed in his lifetime.