WASHINGTON, Aug. 6 (Xinhua) -- The U.S. government is still investigating who leaked the secret warrantless eavesdropping program, which was first revealed by The New York Times in December 2005, The Washington Post reported on Monday.

    Agents from the Federal Bureau of Investigation (FBI) searched the home of former Justice Department lawyer Thomas Tamm last week in an effort to determine who leaked details of the program to the media, according to the newspaper.

    It quoted Newsweek magazine as reporting Sunday that the agents, who had obtained a classified search warrant, took Tamm's desktop computer, two laptops belonging to his children and some of Tamm's personal files.

    Tamm, who left the department last year, had worked in the department's Office of Intelligence Policy and Review, a unit that oversees surveillance of terrorist and espionage targets.

    In December 2005, The New York Times published a story exposing the Bush administration's warrantless wiretapping program to eavesdrop on international phone calls and the e-mails of U.S. residents without court warrants.

    The eavesdropping was conducted without public knowledge and without court approval until January this year, when the program was put under the authority of the Foreign Intelligence Surveillance Court.

    The program, authorized by President George W. Bush shortly after the Sept. 11 attacks, has been criticized by Democrats and some Republicans who believe that Bush may have overstepped his constitutional authority and violated a 1978 law.

    After intensified pressure from the White House, the U.S. Congress last week approved a bill that updates the 1978 Foreign Intelligence Surveillance Act (FISA).

    The new measure is supposed to regulate the way the government can listen to the private communications of American citizens and require a court review of the government's domestic spying program before it could proceed.

    Bush signed the bill on Sunday, which expands temporarily the government's authority to eavesdrop on the international phone calls and e-mail messages of U.S. residents without court warrants.