From The Washington Post:
In the summer of 2011, Secret Service agents responsible for monitoring the White House grounds were reassigned to help protect an agency staffer who was a close friend of the Secret Service director. The agents involved were members of a special unit called Prowler, which is responsible for monitoring the White House perimeter. The unit is especially focused on the South Lawn between 15th and 17th Streets NW, when the president is leaving or arriving via helicopter and motorcade.
