The database was created in 2004 and is maintained by the Terrorist Screening Center, a joint operation between the FBI and the Department of Homeland Security. It includes information from the Transportation Security Administration's air passenger "no-fly" list, the State Department's Consular Lookout and Support System list and the FBI's Violent Gang and Terrorist Organizations File, the Washington Post reported.

The newspaper said last year, some 20,000 "encounters" with people on the list were made by authorities, more than half by Customs and Border Protection officers. Of those, 550 were refused entry or detained, officials said.

"This really confirms the long-standing fear that this list is inaccurate and ultimately ineffective as an anti-terrorism tool," David Sobel, senior counsel with the Electronic Frontier Foundation advocacy group told the Post.

He said the numbers "suggest a staggeringly high rate of false positives with respect to the identification of supposed terrorists."