Testifying before the Senate Judiciary Committee, Mueller described the program as limited. He said the FBI and Justice Department are working out legal ramifications of aerial surveillance.
"I will tell you that our footprint is very small," he said in response to questions from Sen. Chuck Grassley, R-Iowa, the ranking minority member of the committee. "We have very few and have limited use, and we're exploring not only the use but also the necessary guidelines for that use."
Mueller, appointed by former President George W. Bush, became head of the FBI Sept. 4, 2001, a week before the attacks on the World Trade Center and Pentagon. He is scheduled to step down in September after a two-year extension of his current term.
Sen. Dianne Feinstein, D-Calif., who chairs the committee, called drones "the greatest threat to the privacy of Americans."
"Well, it's very seldom used and generally used in a particular incident where you need the capability," Mueller responded.
He said the program "is very narrowly focused on particularized cases and particularized needs and particularized cases. And that is the principal privacy limitations we have."