The FBI sent two responses this month, one classified and one unclassified.

In the unclassified version, the FBI went on the record about the 10 cases of drone surveillance. The FBI also maintained that it would acquire a warrant before using a drone when the suspected individual has a reasonable expectation of privacy.

Paul sent a new letter on Thursday, asking for more clarification on the FBI's understanding of a "reasonable expectation of privacy."

"I am concerned that an overbroad interpretation of this protection would enable more substantial information collection on an individual in a circumstance they might not have believed was subject to surveillance," the letter stated.

He also pressed for copies of educational and training material the FBI uses on such matters.