President Barack Obama is expected to nominate James B. Comey, a former Justice Department official, to be the next director of the FBI, a source said on Wednesday.

If Comey, a Republican, is confirmed by the Senate, he would replace Robert Mueller, who became the FBI’s director only days before the attacks of Sept. 11, 2001. Mueller is expected to step down this fall, Reuters reports. Mueller's term had been set to expire in September 2011 -- FBI directors usually serve for 10 years -- but the president requested that Congress approve a two-year extension, according to CNN.

The White House has not commented on Obama's decision, but the source did say that the president this week had been most interested in Comey for the post, Reuters notes. Lisa Monaco, the White House homeland security adviser who became a key aide to Obama following the Boston Marathon bombings in April, had also been considered for the position, several news outlets report.

Comey, 52, was deputy U.S. attorney general for President George W. Bush. He  previously was the U.S. attorney for the Southern District of New York. Comey testified to a Senate committee in 2007 that he considered resigning from his administration position due to a disagreement over the National Security Agency's domestic surveillance program, according to CNN.  

When he was acting attorney general while John Ashcroft was hospitalized with pancreatitis in 2004, Comey refused to certify legal aspects of a National Security Agency domestic surveillance. Due to the refusal, senior White House officials, counsel Alberto Gonzales and chief of staff Andrew Card attempted to convince Ashcroft to sign the certification. Comey, who was in the room, said Ashcroft refused, according to news reports.

At a 2007 hearing, Comey told the Senate Judiciary Committee that the incident was "probably the most difficult night of my professional life." Comey’s actions gained the admiration of Democrats who were opposed to the Bush administration’s domestic surveillance program, Reuters says.

Earlier in his career, while he was assistant U.S. attorney for the Eastern District of Virginia, Comey dealt with the Khobar Towers bombing case, which came after a 1996 attack on a U.S. military facility in Saudi Arabia, in which 17 American military members were killed.