Among those was suspected Boston Marathon bomber Tamerlan Tsarnaev, whose name was added in 2011. The increase in names is due in part to security agencies using the system more in the wake of the failed 2009 attack on a plane by "underpants bomber" Umar Farouk Abdulmutallab in Detroit.
Intelligence and law enforcement officials acknowledged in Congress that they had missed clues to that attack despite Abdulmutallab's name appearing in the main database, known as TIDE.
Maintained by the National Counterterrorism Center, the highly classified database is not a "watchlist" but instead is a repository of information on people whom U.S. authorities see as known, suspected or potential terrorists from around the world.
The "Terrorist Identities Datamart Environment" is a master database which agencies use to build other catalogs of possible terrorists, like the "no-fly" list which prevents people who feature on it from boarding airplanes.