 reportedly had word for some time about the 20-year-old gunman acting suspiciously before he opened fire on the roof of that building 150 yards away from the former president. Director of the United States Secret Service, Kimberly Cheeto, speaks exclusively with ABC News, taking responsibility for that stunning breakdown in security. This is an event that should have never happened. Who is most responsible for this happening? What I would say is that the Secret Service is responsible for the protection of the former president. So the buck stops with you?