The Terrorist Screening Database was created in 2003 by order of a Homeland Security Presidential Directive. The database includes the names and aliases of anyone known to be, or reasonably suspected of being, involved in terrorism or assisting terrorists through financial aid or other ways. The federal Terrorist Screening Center maintains the database, and an array of government agencies nominate people to it through the National Counter Terrorism Center.
Some of the information in the database originates with the Terrorist Identities Datamart Environment, also called TIDE. That list contains classified data collected by intelligence agencies and militaries worldwide, but anything passed on to the terrorist watch list is first scrubbed of classified info. In 2013, TIDE had 1.1 million names in it.
The State Department checks all visa applicants against the watch list. The TSA’s No-Fly list and Selectee List, which identifies people who warrant additional screening and scrutiny at airports and border crossings, are also derived from the watch list. But it is most often used by law enforcement agencies at all levels to check the identity of anyone arrested, detained for questioning, or stopped for a traffic violation. The FBI calls it “one of the most effective counterterrorism tools for the US government.”
Entries in the database are coded according to threat level to provide law enforcement with instructions on what to do when they encounter a suspected terrorist who is on the list. According to a 2005 inspector general report (.pdf), of some 110,000 records in the database that the IG reviewed, 75 percent of them were given handling code 4, considered the lowest level, and 22 percent were given handling code 3. Only 318 records had handling codes 1 or 2. A description of what each level means is redacted in the publicly released version of the document, but a note indicates that people are usually given code 4 when they are either just an associate of a suspected terrorist and therefore may not pose a threat or if there is too little information known about the individual to categorize them at a higher level.
Appearing in the database doesn’t mean you’ll be arrested, denied a visa, or barred from entering the country. But it does mean your whereabouts and any other information gleaned from, say, a traffic stop, will be added to the file and scrutinized by authorities.