Investigative filmmaker Michael Kirk tells the inside story of how the government came to spy on millions of Americans—from September 11, 2001, up through the present. His Frontline documentary “United States of Secrets” explores the secret history and scope of the government's unprecedented post-9/11 surveillance program (and the lengths the government went to keep the program secret from the public). “United States of Secrets” premieres May 13 on PBS.