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WASHINGTON - Attorney General Eric Holder on Tuesday fended off Republican demands that he appoint a special counsel to look into national security leaks.

Holder said he and FBI Director Robert Mueller have already been interviewed by the FBI as part of a Justice Department investigation.

At a Senate Judiciary Committee hearing, Sens. Lindsey Graham, R-S.C., and Chuck Grassley, R-Iowa, said they want the attorney general to appoint a special counsel, rather than Holder's choices, U.S. Attorneys Ron Machen and Rod Rosenstein, political appointees.

Holder praised the two, appointed to oversee investigations into who leaked information about U.S. involvement in cyberattacks on Iran and an Al-Qaida plot to put an explosive device aboard a U.S.-bound flight.

"We have people who have shown independence, an ability to be thorough and who have the guts to ask tough questions," Holder said. "And the charge that I've given them is to follow the leads wherever they are."

Republicans interspersed their criticism of the leak probes with criticism of his failure to turn over more documents to a House committee investigating a flawed gun-smuggling probe in Arizona, Operation Fast and Furious.

Sen. John Cornyn called on Holder to resign, saying the attorney general had misled Congress in February by embracing a letter to Congress denying there were problems. Hundreds of illicitly purchased weapons wound up south of the border, many of them at crime scenes.

Holder offered to sit down with Republicans to resolve the impasse over the documents.