The Federal Bureau of Investigation (FBI) says it has used drones for domestic surveillance purposes in the United States at least ten times without obtaining warrants. In three additional cases, drones were authorized but “not actually used.”

Sen. Rand Paul (R-Ky.) on Thursday published a letter from FBI Assistant Director Stephen D. Kelly, who admitted that the agency used unmanned aerial vehicles (UAVs) domestically, without gathering any warrants.

“The FBI uses UAVs in very limited circumstances to conduct surveillance when there is a specific, operational need,” the letter reads. “Since late 2006, the FBI has conducted surveillance using UAVs in eight criminal cases and two national security cases.”

The bureau said that it would only be required to obtain a warrant to use a drone in cases for which a person “would have a reasonable expectation of privacy.” The FBI stated that it has not yet needed to ask for a warrant, but that all requests for drone use must be reviewed by an agency lawyer and approved by a senior management official.

The agency said that one of the cases involved the rescue of a five-year-old boy who was being held hostage in an underground bunker. The information strongly suggests that the agency was referring to the Alabama hostage crisis in which a retired truck driver kidnapped a boy from a school bus and held him hostage for six days.

Drone usage was also authorized in three additional cases, but the FBI did not release details about the nature of those circumstances.

Sen. Paul has long advocated against domestic drone usage, and in March held up CIA Director John Brennan’s nomination for nearly 13 hours, due to his history of defending drone strikes. In June, FBI Director Robert Mueller admitted that his agency uses drones to spy on US citizens without any“operational guidelines.” Since Mueller’s announcement, Paul has sent a series of open letters to the FBI, requesting detailed information about its use of drones in the United States.

In his second letter, mailed July 9, Paul threatened to filibuster the confirmation hearing for James Comey, Mueller’s successor, if he didn’t receive a response. This week, Paul finally received the information he sought.

But Paul was discontent with part of the agency’s response. In a follow-up letter addressed to Mueller, he expressed concern about the FBI’s ability to use drones without a warrant in cases where there is no“reasonable expectation of privacy.” He said the clause could result in “an over-broad interpretation of this protection,” and asked the FBI to clarify what would require a warrant.

Sen. Paul also took his concerns to Twitter, telling his followers that “spying without warrants is unconstitutional.”

Overall, confirmation of the FBI’s drone use might be cause for concern among privacy advocates and anti-drone activists. Dave Norris, a councilman of Charlottesville, Va., predicted last February that drone use would occur domestically, and feared that there would be room for abuse.

“To me, it’s Big Brother in the sky,” he told the New York Times. “I don’t mean to sound conspiratorial about it, but these drones are coming, and we need to put some safeguards in place so they are not abused.”