The FBI uses drones for surveillance of stationary subjects, and the privacy implications of such operations are "worthy of debate", the bureau's director has said.
Robert Mueller said the law enforcement agency seldom uses drones now, but is developing guidelines that will shape how unmanned aerial vehicles are used.
There will be a number of issues regarding drones "as they become more omnipresent, not the least of which is the drones in air space and also the threat on privacy", Mr Mueller said in an appearance before the Senate Judiciary Committee. "We already have, to a certain extent, a body of law that relates to aerial surveillance and privacy relating to helicopters and small aircraft ... which could well be adapted to the use of drones. It's still in its nascent stages ... but it's worthy of debate and perhaps legislation down the road."
Drones "allow us to learn critical information that otherwise would be difficult to obtain without introducing serious risk to law enforcement personnel," the FBI added in a statement.
The FBI used drones at night during a six-day hostage stand-off in Alabama earlier this year. The siege ended when members of an FBI rescue team stormed an underground bunker, killing gunman Jimmy Lee Dykes before he could harm a five-year-old boy being held hostage.
The FBI said its unmanned aerial vehicles are used only to conduct surveillance operations on stationary subjects. In each instance, the FBI first must obtain the approval of the Federal Aviation Administration to use the aircraft in a very confined area.
The aerospace industry forecasts a worldwide deployment of almost 30,000 drones by 2018, with the United States accounting for half.
Mr Mueller also urged Congress to move carefully before making any changes that might restrict the National Security Agency programmes for mass collection of people's phone records and information from the internet. He said there are 10 or 12 cases in which the phone records programme contributed to breaking up terrorist plots.
Mr Mueller said communications capabilities of terrorists are their weakest link. "If we are to prevent terrorist attacks, we have to know and be in their communications. Having the ability to identify a person in the United States, one telephone number with a telephone that the intelligence community is on in Yemen or Somalia or Pakistan ... may prevent that one attack, that Boston or that 9/11."
The FBI director argued for the continued use of the NSA programmes. "Are you going to take the dots off the table, make it unavailable to you when you're trying to prevent the next terrorist attack? That's a question for Congress," said Mr Mueller.