In the summer, a Navy drone, experiencing what the military called a software problem, wandered into restricted Washington airspace. Last month, a small Mexican army drone crashed into a residential yard in El Paso.
There are also regulatory issues with civilian agencies using military frequencies to operate drones, a problem that surfaced in recent months and has grounded the Texas DPS drones, which have not been flown since August.
"What level of trust do we give this technology? We just don't yet have the data," said John Allen, director of Flight Standards Service in the FAA's Office of Aviation Safety. "We are moving cautiously to keep the National Airspace System safe for all civil operations. It's the FAA's responsibility to make sure no one is harmed by [an unmanned aircraft system] in the air or on the ground."
Officials in Texas said they supported the FAA's concern about safety.
"We have 23 aircraft and 50 pilots, so I'm of the opinion that FAA should proceed cautiously," Nabors said.
Much of the legal framework to fly drones has been established by cases that have examined the use of manned aircraft and various technologies to conduct surveillance of both public spaces and 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."
At least one community has already balked at the prospect of unmanned aircraft.
The Houston Police Department considered participating in a pilot program to study the use of drones, including for evacuations, search and rescue, and tactical operations. In the end, it withdrew.
A spokesman for Houston police said the department would not comment on why the program, to have been run in cooperation with the FAA, was aborted in 2007, but traffic tickets might have had something to do with it.
When KPRC-TV in Houston, which is owned by The Washington Post Co., discovered a secret drone air show for dozens of officers at a remote location 70 miles from Houston, police officials were forced to call a hasty news conference to explain their interest in the technology.
A senior officer in Houston then mentioned to reporters that drones might ultimately be used for recording traffic violations.
Federal officials said support for the program crashed.
Staff researcher Julie Tate contributed to this report.