WASHINGTON, Aug. 25 (Xinhua) -- The U.S. government's terrorist screening database flagged Americans and foreigners as suspected terrorists almost 20,000 times last year, but only a small fraction of those questioned were arrested or denied entry into the United States, The Washington Post reported Saturday.

    Slightly more than half of the 20,000 encounters last year were logged by Customs and Border Protection officers, who turned back or handed over to authorities 550 people, most of them foreigners, the newspaper quoted Customs officials as saying.

    Few specifics are known about how the system operates, how many people are detained or turned back from borders, or the criteria used to identify suspects. The government would not discuss cases, nor would it confirm whether an individual's name was on its list, the report said.

    FBI and other officials said that they could not provide data on the number of people arrested or denied entry for the other half of the database hits, but FBI officials indicated that the number of arrests was small.

    A range of state, local and federal agencies as well as U.S. embassies overseas rely on the database to pinpoint terrorism suspects, who can be identified at borders or even during routine traffic stops, the report said.

    The watch list, maintained by the Terrorist Screening Center, a joint operation between the FBI and the Department of Homeland Security, 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.

    According to the Justice Department's inspector general, the database contained at least 235,000 records as of last fall.

    The government says the database is a powerful tool for identifying and tracking suspected terrorists and for sharing intelligence, and that its purpose is not necessarily to make arrests. But the new details about the numbers, disclosed in an FBI budget document and in interviews, raise questions about the database's effectiveness and its impact on privacy, critics were quoted as saying.