(CNN) — - Sen. Rand Paul is vowing to block a confirmation vote on the next FBI director unless he gets sufficient answers from the law enforcement agency about the use of surveillance drones in the United States.
The FBI revealed in a recently unclassified letter to the Kentucky Republican that it has used unmanned aerial vehicles for surveillance in 10 cases on U.S. soil - eight of them criminal and two involving national security.
"None of the UAVs (unmanned aerial vehicles) used by the FBI are armed with either lethal or non-lethal weapons, and the FBI has no plans to use weapons with UAVs," wrote FBI Assistant Director Stephen Kelly.
Drones were authorized for surveillance in three other criminal cases, but they were not used, according to the letter.
While CNN previously reported the FBI has used drones on U.S. soil about a dozen times, the recent letter marked the first time it broke the number down between criminal cases and national security cases.
The letter listed examples of drone use, including in the case of a 5-year-old child held hostage in an underground bunker in Alabama earlier this year.
But Paul says the new information is not enough to answer his questions.
The first-term senator has been a leading voice on seeking answers on the legal use of drones by the U.S. government in counterterror operations overseas.
He led a filibuster earlier this year, blocking a vote on President Barack Obama's nominee of John Brennan as CIA director, while he demanded answers about the government's policy for drone strikes in the United States.
Raising questions now about drone surveillance, Paul issued a letter last month asking FBI Director Robert Mueller about its policies. Getting no response, he sent a second letter on July 9, saying he will object to the consideration of James Comey as Mueller's successor until he gets "adequate answers" to his questions.
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.