While the nation remembers the terrorist attacks that killed thousands of Americans 20 years ago, New York City firefighters, sent to rescue victims at the World Trade Center who survived, will relive a life-changing experience that's now a part of who they are. "It's a day that will never leave you," Fire Department of New York Commissioner Dan Nigro tells Scott Pelley. Nigro and other firefighters at Ground Zero, many of whom fill the top ranks of the FDNY, recall the men, their sacrifices, and the tragedy of losing 343 of their colleagues on the 54th Season Premiere of 60 MINUTES. This one segment runs the full hour.