There were multiple security failures that led to the disaster on 9/11/01. Inadequate passenger screening at Boston’s Logan Airport was one of the last of them, so now we all remove our belts and shoes for the TSA.
Before that fateful day, there was a daily intelligence briefing with a late warning about Osama bin Laden. A warning that George W. Bush chose to minimize, if not ignore.
Much earlier than that, there was an appalling lack of coordination between the FBI and the CIA. So now we have Fusion Centers, as explained last night on the PBS NewsHour.