From 2004 to May 2013, the Justice Department spent almost $5 million on the unmanned aircraft.
In June, then-FBI Director Robert Mueller told Congress that the FBI occasionally uses the unmanned aerial vehicles but was developing guidelines in anticipation of issues that will arise "as they become more omnipresent." In one instance earlier this year, the FBI used drones at night during a six-day hostage standoff in Alabama.
In a letter in July to Sen. Rand Paul, R-Ky., the FBI revealed it had used drones 10 times since 2006 for surveillance in kidnappings, search and rescue missions, and drug and fugitive investigations. Among them was last winter's standoff between authorities and Jimmy Lee Dykes, who was shot to death after holding a 5-year-old boy hostage in an underground bunker in Alabama, the letter said.
The IG's report cited the Alabama case, but no others, saying only that a review of available records showed that the FBI appeared to be operating drones only after obtaining required approvals from the Federal Aviation Administration.
Civil liberties groups critical of domestic drone use say such operations could invade people's privacy. The government worries drones could collide with passenger planes or crash, concerns that have slowed more widespread adoption of the technology.
Paul, mentioned as a possible 2016 GOP presidential candidate, had been thwarting the Senate confirmation vote of Mueller's successor, James Comey, over his concerns about the FBI's domestic use of drones and had asked the FBI to address his concerns.
The FBI's letter to Paul also said that while the Supreme Court had not ruled on the use of drones, prior rulings on aerial surveillance held that court warrants were not needed because the areas monitored were open to public view and "there was no reasonable expectation of privacy."
The agency also wrote that a warrant would not be needed because drones don't physically trespass on private property.
Civilian versions of unmanned military aircraft that have tracked and killed terrorists in the Middle East and Asia are in demand by police departments and border patrol units. Justice Department officials told the IG's office that none of their drones were armed.