US justice authorities are facing calls for new rules on drone surveillance after it was revealed the FBI has been deploying unmanned aircraft since 2006.
A report by the Department of Justice’s watchdog said the FBI has spent $3m (£1.9m) on drone deployment.
The audit is the first time the full extent of drone use in US law enforcement has been acknowledged.
It comes amid privacy concerns and criticism of President Barack Obama over surveillance and drone activity.
Four US Justice Department units, including the FBI, spent $3.7m acquiring and testing drones between 2004 and May this year, the report by the department’s inspector general revealed.
None of the unmanned aircraft was armed or carried “releasable projectiles,” the report said.
The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives have developed or are developing guidelines on drone use, it added.
By relying on existing rules governing manned aircraft use, the inspector general said that the FBI risked violating privacy rights.
The report urged new protocols to restrict improper surveillance.
Drones “can be used in close proximity to a home and, with longer-lasting power systems, may be capable of flying for several hours or even days at a time, raising unique concerns about privacy and the collection of evidence,” it said.
The Justice Department said it had agreed with the watchdog’s recommendations.
A statement from the American Civil Liberties Union said it welcomed the report and called on authorities to take action.
“No agency, including the FBI, should deploy domestic surveillance drones without first having strong privacy guidelines in place,” Jay Staley, the group’s senior policy analyst, said.
“We urge the Justice Department to make good on its plans to develop privacy rules that protect Americans from another mass surveillance technology,” he added.
In June, Robert Mueller, then director of the FBI, told the US Senate’s Judiciary Committee that US law enforcement agencies were using drones in some circumstances.
The FBI said later that unmanned aircraft were only used to monitor stationary subjects and to avoid serious risk to law officers.
The Obama administration has come under increasing criticism for using drones overseas – chiefly to carry out deadly missile attacks against suspected militants in Pakistan, Yemen, Afghanistan, and Iraq.
It has also had to defend its surveillance activities after a series of leaks by former National Security Agency contractor Edward Snowden claimed the US was able to eavesdrop on vast amounts of electronic communications with little oversight.