The Christmas Day attack suspect was on a terrorist watchlist but still managed to board a US-bound flight. The reason lies in the three-tier system of US watchlists.
The US government maintains a multi-level system of watch lists to try and identify possible terrorists before they enter the country.
These lists have come under intense scrutiny in the wake of the failed attempt to blow up an airliner bound for Detroit on Christmas Day. The system contained the name of the alleged bomber in the case, Umar Farouk Abdulmutallab. Yet he yet still managed to fly from Nigeria, through Amsterdam, to the US.
The entry-level, broadest list of possible terrorist names is the Terrorist Identities Datamart Environment (TIDE) database, which is run by the National Counterterrorism Center and contains well over 500,000 entries.
Basically, this is a list of suspects. US law enforcement and intelligence agencies – plus their foreign counterparts – submit names to this database every day.
According to an NCC fact sheet, the sorts of activity that can get on included in TIDE are everything from possible terrorist activity to the provision of material support for terrorists, such as safe houses, communications, or money.