The director of the U.S. Federal Bureau of Investigation has contradicted sworn testimony by Attorney General Alberto Gonzales about a secret U.S. surveillance program.

FBI Director Robert Mueller said Thursday that the legality of the surveillance program was the subject of intense debate among justice officials before it was publicly revealed in 2005. The program allowed wiretapping of some international phone calls originating or terminating in the United States.

Gonzales, who is Mueller's boss, has testified that the program prompted no disagreements within the administration.

This is the latest blow to the attorney general's struggle to stay in office. Senate lawmakers have been calling on him to step down over the legality of the surveillance program and his role in the firings of several federal prosecutors last year.

Many lawmakers contend the prosecutors' dismissals were politically motivated, and some have accused the White House of direct involvement. The Bush administration has repeatedly denied the allegations.

Thursday, four Democratic senators called for an independent investigation into allegations that Gonzales lied under oath while testifying about the firings.

Also Thursday, the U.S. Senate Judiciary Committee issued subpoenas for White House aides Karl Rove and Scott Jennings as part of an investigation into the firings.

Committee Chairman Democratic Senator Patrick Leahy said the fired prosecutors resisted attempts to influence political corruption cases to benefit Republican candidates.

President Bush has already refused to permit testimony on the issue from other White House officials. He has told Congress that he has the right to withhold public testimony by those who offer him private advice.