It’s been 25 years since al-Qaida attacked the United States by hijacking airplanes and crashing them into the World Trade Center and the Pentagon, with a fourth plane crashing into a field in Pennsylvania.Sept. 11 represented a new kind of national security threat, and a new national security architecture was created to confront it.Before a live audience at NPR, host Mary Louise Kelly discusses how that day changed the country and its spy agencies, with people who worked on the frontlines of government and intelligence.Email the show at sourcesandmethods@npr.org.NPR+ supporters hear every episode without sponsor messages and unlock access to our complete archive. Sign up at plus.npr.org.