An unmanned Predator drone flies over Kandahar Air Field, southern Afghanistan, on a moonlit night on Jan. 31, 2010. Drones are also being used by the United States in Pakistan, Libya, and Yemen. Kirsty Wigglesworth/AP/File
Staff Sgt. Christopher DeLucia checks systems on an unmanned aircraft at Creech Air Force Base in Nevada. The US has 120 drones, 35 aloft at any given time. Tony Avelar/The Christian Science Monitor
A Predator drone armed with a missile stands on the tarmac of Kandahar military airport in Afghanistan on June 13, 2010. Massoud Hossaini/Pool/Reuters/File
Captain Bob, a US Air Force officer who asked not to be identified fully, operates a Predator unmanned aircraft system (UAS) over Afghanistan from a control station at an Air Force base in Nevada. Tony Avelar/The Christian Science Monitor
United States Air Force Maj. Sgt. Lyle (l.) wears a 432nd Wing Shield on his arm as he operates a Predator unmanned aircraft system over Kandahar, Afghanistan, from a ground control station at Creech Air Force Base in Indian Springs, Nevada. Tony Avelar/The Christian Science Monitor
United States Air Force Capt. Bob operates a Predator unmanned aircraft system over Kandahar, Afghanistan, from Indian Springs, Nevada. Tony Avelar/The Christian Science Monitor
An airman assigned to the 432nd Aircraft Maintenance Squadron works on the front end of a Predator UAS at Creech Air Force Base. Tony Avelar/The Christian Science Monitor
Capt. Bob and Major Sgt. Lyle (l.) step out of a mobile conex container after finishing their shift operating a Predator UAS over Kandahar, Afghanistan. Tony Avelar/The Christian Science Monitor
Airmen roll out from the hanger a Predator UAS at Creech Air Force Base in Indian Springs, Nevada. After being launched overseas, the Predators are flown by pilots and sensor operators from ground control stations at Creech and four other Air National Guard units in the US. In addition to carrying two laser-guided Hellfire missiles, the Predators have cameras that provide full-motion video that is in high demand for use in intelligence, surveillance, and reconnaissance. Tony Avelar/The Christian Science Monitor
Graduate students Tuna Tuksoz (l.) and Mark Cutler in MIT's Department of Aeronautics and Astronautics work on a simulator of a winged aircraft - a quadrotor - that has military applications, in Cambridge, Mass., Sept. 27, 2011. Melanie Stetson Freeman/Staff
Demonstrators protest against the use of drones prior to the second US presidential campaign debate in Hempstead, New York, October 16, 2012. Lucas Jackson/Reuters
Sgt. Scott Weaver, of the Queen's Royal Lancers, is seen launching a Black Hornet Nano unmanned air vehicle from a compound in Afghanistan. British troops in Afghanistan are the first to use the small surveillance helicopters, which measure 10 by 2.5 centimeters and are equipped with a tiny camera that gives troops video and still images. Sgt Rupert Frere/RLC/MoD/Reuters
An Aeryon Scout unmanned aerial vehicle hovers at the Nome causeway in Alaska. The drone glides on 20-minute missions ranging from 10 to 320 feet above the ice, and its images can be instantly viewed on a tablet-type computer screen. University of Alaska Geophysical Institute/AP
FBI Director Robert Mueller told the Senate Judiciary Committee Wednesday that the FBI is using drones to conduct secret surveillance on US citizens. Many lawmakers seemed surprised.
Even as President Obama was calling for prudence in the use of drones Wednesday to an audience in Berlin, over on Capitol Hill came new revelations that the Federal Bureau of Investigation has been using drones to conduct secret surveillance on US citizens.
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