 Matt Piper, CBS News, Milwaukee. Nikki Haley is expected to speak at the convention today. Homeland Security Secretary Alejandro Mayorkas acknowledges Saturday's assassination attempt on Mr. Trump at a campaign rally in Pennsylvania was a security failure. I do intend to speak with members of the Hill with respect to the resources that we need. Cell phone video shows rally attendees pointing toward the gunman, Thomas Matthew Crooks, trying to alert police two minutes before he opened fire.