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A flag reading "Never Forget 9/11" at the National 9/11 Memorial and Museum in New York, on Aug. 31. Photo: Angela Weiss / AFP via Getty Images
Twenty-five years after the Sept. 11 attacks, some lessons remain deeply ingrained in the national security apparatus — while others have faded.
The big picture: A vast majority of Americans say the attacks forever changed the country. But some key post-9/11 security norms — like airport screening, intelligence gathering and international cooperation — have evolved with new technology and shifting political whims.
