The number of names on a highly classified U.S. central database used to track suspected terrorists has jumped to 875,000 from 540,000 only five years ago, a U.S. official familiar with the matter said.
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.
The official familiar with the latest statistics said that even though the number of TIDE entries has grown substantially, this does not mean that the data is less manageable as intelligence agencies have gotten better at figuring how to extract information from the oceans of data.
However, Karen Greenberg, an expert in counter-terrorism policy at Fordham University, questioned whether the growth in the database's size made it easier for officials to spot threats before they materialize.
"What you want is more focus, not less focus. It can't be just about quantity. It has to be about specificity," she said.
The vast size of TIDE came into the spotlight in the wake of the bombing late last month of the Boston Marathon.
U.S. officials now acknowledge that Tamerlan Tsarnaev's name was entered into TIDE by the CIA in the autumn of 2011, after the U.S. spy agency received a request from Russian authorities to investigate him for suspected radical Islamist activities.
The CIA also entered the name of Tsarnaev's mother, Zubeidat Tsarnaeva's, into the TIDE database. Tamerlan Tsarnaev died in a shootout with authorities in the days following the Boston bombing and his younger brother Dzhokhar was captured by police.