Republican Sen. Rand Paul called on the FBI Thursday to release more information on its use of unarmed drones inside the United States.

In a letter to outgoing FBI director Robert Mueller, Paul said he was "disturbed" that federal law enforcement officials agreed to use drone surveillance domestically without a "governance policy." In his final time appearing before the Senate Judiciary Committee, Mueller on Wednesday revealed that the FBI used unmanned aircraft to conduct surveillance, albeit in "minimal way," he said.

"Our footprint is very small," Mueller said. "We have very few."

Paul, R-Ky., asked in his letter for more information on the FBI's domestic drone program and how long it had been in operation. He also wants to know whether the drones "have the capacity to be armed" and if the director foresaw the U.S. using armed drones domestically.

"In the future," Paul wrote, "I hope that your agency intends to be more forthcoming with information on its use of drone surveillance."

The Kentucky lawmaker gained national notoriety in March for his 13-hour filibuster on the Senate floor against the use of drone strikes within the U.S. Since then, the libertarian-minded Paul has become a favorite among conservatives and Tea Party groups, who adopted the slogan "Stand with Rand."

President Obama is set to nominate former Justice Department official James Comey Friday to replace Mueller as head of the FBI.