Officially, the United States does not use its significant fleet of unmanned drones for surveillance on US soil. But a clause in the Air Force's policy on accidentally collected surveillance is raising eyebrows. According to the guidelines given to operators, photos and video of US citizens taken without their consent can be kept for up to 90 days, in which it will be analyzed to determine whether or not it can be kept under current domestic spying laws.
The wording of this directive was discovered by Steven Aftergood of the Federation of American Scientists. The concern that Mr. Aftergood and others share is that the Air Force can accidentally spy on US citizens, then leisurely investigate both the footage and the individuals and property therein to determine whether or not the spying was justified. Even if you avoid the cynical proposition that an Air Force engineer might be tempted to "accidentally" leave the cameras rolling during a test flight over a sensitive area, the idea of investigating evidence for wrongdoing after it's been collected is something like justifying a search warrant with the drugs that a DEA agent discovered after illegally entering a private residence.
If the evidence collected by the Air Force is indeed found to incriminate US citizens, it can be shared with law enforcement agencies with a court order. The Air Force's domestic interests only extend to terrorist activities and large-scale drug enforcement, but there's no stipulation of what purpose the surveillance has to serve if it's passed around. This creates even more accountability problems, since there's technically no way that law enforcement should be privy to information gathered by the military in a surveillance capacity. The way that the guidelines are written now, not-quite-accidental surveillance without any warrant or court oversight is a disturbingly possible reality.