But by 2013, the FAA expects to have formulated new rules that would allow police across the country to routinely fly lightweight, unarmed drones up to 400 feet above the ground - high enough for them to be largely invisible eyes in the sky.
Such technology could allow police to record the activities of the public below with high-resolution, infrared and thermal-imaging cameras.
One manufacturer already advertises one of its small systems as ideal for "urban monitoring." The military, often a first user of technologies that migrate to civilian life, is about to deploy a system in Afghanistan that will be able to scan an area the size of a small town. And the most sophisticated robotics use artificial intelligence to seek out and record certain kinds of suspicious activity.
But when drones come to perch in numbers over American communities, they will drive fresh debates about the boundaries of privacy. The sheer power of some of the cameras that can be mounted on them is likely to bring fresh search-and-seizure cases before the courts, and concern about the technology's potential misuse could unsettle the public.
"Drones raise the prospect of much more pervasive surveillance," said Jay Stanley, a senior policy analyst with the American Civil Liberties Union's Speech, Privacy and Technology Project. "We are not against them, absolutely. They can be a valuable tool in certain kinds of operations. But what we don't want to see is their pervasive use to watch over the American people."
The police are likely to use drones in tactical operations and to view clearly public spaces. Legal experts say they will have to obtain a warrant to spy on private homes.
In a 1986 Supreme Court case, justices were asked whether a police department violated constitutional protections against illegal search and seizure after it flew a small plane above the back yard of a man suspected of growing marijuana. The court ruled that "the Fourth Amendment simply does not require the police traveling in the public airways at this altitude to obtain a warrant in order to observe what is visible to the naked eye."
In a 2001 case, however, also involving a search for marijuana, the court was more skeptical of police tactics. It ruled that an Oregon police department conducted an illegal search when it used a thermal imaging device to detect heat coming from the home of an man suspected of growing marijuana indoors.
"The question we confront today is what limits there are upon this power of technology to shrink the realm of guaranteed privacy," Justice Antonin Scalia wrote in the 2001 case.
Still, Joseph J. Vacek, a professor in the Aviation Department at the University of North Dakota who has studied the potential use of drones in law enforcement, said the main objections to the use of domestic drones will probably have little to do with the Constitution.
"Where I see the challenge is the social norm," Vacek said. "Most people are not okay with constant watching. That hover-and-stare capability used to its maximum potential will probably ruffle a lot of civic feathers."