Sen. Rand Paul, left, wants FBI Director Robert Mueller to come clean about domestic drones. (Getty Images)

Sen. Rand Paul, R-Ky., threatened Tuesday to filibuster the nomination of James Comey to lead the Federal Bureau of Investigation if current FBI Director Robert Mueller fails to produce answers about the bureau's use of drones within the United States.

Comey, a deputy attorney general during the George W. Bush administration, was expected to win easy confirmation. During a Senate Judiciary Committee hearing Tuesday Sen. Orrin Hatch, R-Utah, told the nominee, "I would be surprised if this wasn't the third time you were unanimously approved by the Senate."

A full Senate vote on Comey's nomination has not been scheduled.

In March Paul and a cohort of like-minded senators stood on the Senate floor for 13 hours to filibuster the nomination of John Brennan to lead the CIA after Attorney General Eric Holder failed to definitively rule out using drones to kill people within the U.S. Paul stressed at the time that he wasn't personally targeting Brennan, but using the opportunity to get answers to an important question.

Comey became a hero to civil libertarians for refusing to reauthorize a domestic surveillance program in 2004.

Paul initially asked Mueller to explain the FBI's use of drones in a June 20 letter after Mueller told the Senate Intelligence Committee his agency was using the unmanned devices without clear guidelines.

The likely 2016 presidential candidate asked Mueller how long the FBI has been using drones, how many drones the FBI has in its inventory, whether or not FBI drones would ever be armed, why they are used, what policies guide their use and what has been done with the information they collect.

"I indicated that I would like a response to my questions by July 1, 2013, which was a very reasonable timeframe to produce a response to a limited number of questions," Paul said in a follow-up letter sent Tuesday. "Legitimate questions on important government functions should not be ignored."

"The President has submitted the nomination of your successor to the Senate, the Senate Judiciary Committee has begun consideration of his nomination, and that nomination could be considered by the full Senate this month," Paul added. "Without adequate answers to my questions, I will object to the consideration of that nomination and ask my colleagues to do the same."

It's unclear why the FBI did not immediately provide answers. FBI spokesman Paul Bresson told U.S. News on Friday that the bureau is "still in the process of responding" to Paul's letter.