The Pentagon has deployed drones several times over U.S. territory on non-military spy missions, but the flights have been rare and lawful, according to a Defense Department report obtained by USA Today under the Freedom of Information Act.

The report, by the Defense Department inspector general, said the Pentagon had flown drones over U.S. territory fewer than 20 times between 2006 and 2015, always in compliance with existing U.S. law, USA Today reported on Wednesday.

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Use of aerial surveillance drones over the United States became public in 2013 when the FBI director Robert Mueller testified before Congress that the bureau employed spy drones during investigations on rare occasions, USA Today said.