Before it started inflicting full-body pat downs on passengers who declined to pass through full body scanners at some domestic airports, the Transportation Security Administration decided to allow some terrorists to board airplanes unscreened because, among other things, it did not want to inconvenience travelers.
It also did not want to tip off some terrorists who were not deemed a threat to the airplane and who were under surveillance.
When Congress funded the Homeland Security Department for fiscal 2009, it required TSA to certify to the House and Senate appropriations committees whether it was going to use the government's full non-classified terrorist watch list -- the Terrorist Screening Database (TSDB) -- to screen passengers boarding airplanes.
The law said that if the TSA determined it "does not need to check airline passenger names against the full terrorist watch list, then the Assistant Secretary shall certify to the Committees on Appropriations of the Senate and the House of Representatives that no significant security risks are raised by screening airline passenger names only against a subset of the full terrorist watch list."
What is the full terrorist watch list, and where does it come from? As set out by a presidential directive issued in 2003, the TSDB includes "individuals known or reasonably suspected to be or have been engaged in conduct constituting, in preparation for, in aid of, or related to terrorism."
The TSDB sits in the middle of an inverted pyramid of government lists of known and suspected terrorists. At the broad top of this inverted pyramid is the Terrorist Identities Datamart Environment (TIDE), maintained by the National Counterterrorism Center, under the director of national intelligence. TIDE includes the identities and derogatory information about every foreign national reasonably suspected of being a terrorist, plus identities of their family members.
The TSDB is one step down from TIDE. It is maintained by the Terrorist Screening Center, a division of the FBI. The TSDB includes the names of people from TIDE who are "known or reasonably suspected of being terrorists" who can be positively identified at places such as airport counters. It also includes the names of a few U.S. citizens, nominated by the FBI.
The Selectee and No Fly lists are subsets of the TSDB. The Selectee list includes "known or suspected" terrorists TSC believes should be set apart by the TSA for enhanced security screening when boarding airplanes, and the No Fly list includes "known or suspected" terrorists TSC believes should not be allowed to board a plane, period.