WASHINGTON, Dec. 30 (UPI) -- The Justice Department says it is investigating leaks about the National Security Agency's classified domestic surveillance program.

The secret eavesdropping program, which President George Bush authorized shortly after the Sept. 11, 2001, attacks, allows the NSA to intercept domestic communications without a warrant, so long as one party is outside the United States.

Bush, who confirmed the program's existence earlier this month after The New York Times (NYSE:NYT) broke the story, says it is essential to help counterterrorism agents quickly trace the communications of terror suspects.

Democrats and some Republicans have questioned the legality of the program and some are seeking an independent investigation or congressional hearings.

During a recent news conference Bush suggested there may be an investigation into the leak of information about the program. The Times said it held the story -- at the government's request -- for about a year before publishing. One of the Times reporters plans a book on the issue to come out early next year.