There are two possibilities here:
Kennedy often used the Secret Service to take himto and from his meeting places with MarylinMonroe, who he was carrying out an affair with
The day before the parade, the route was changed toincorporate the turn onto Elm St.
The head of the Secret Service was sent out of thecountry
In the book Mortal Error: The Shot That KilledJFK, the author Bonar Menninger claims that SecretService agent George Hickey accidentally killedJFK while attempting to return fire on the gunman