In 1950, two Puerto Rican nationalists attempted to kill Pr. Harry Truman. Truman was living in Blair House at the time while the White House was being renovated. They attacked at 2 PM, opening fire on the guards. Training, tactics and weapons were not really adequate, but the Secret Service and White House Police managed to prevail.
One officer was killed, two others wounded. The gunfight lasted about 40 seconds.
I found the a write-up of the events on the Family, Friends and Firearms Forum. If you scroll down in this link, the full article by Massad Ayoob from American Handgunner Magazine is posted. His review and assessment of the events is worth reading.
When things go wrong, they go all the way wrong, and it happens very fast.
UPDATE: TheAxe left us a comment recommending a book about the assassination attempt, it's called American Gunfight, Stephen Hunter is the author.