In a heavily protected military base some 15 miles south of Washington, D.C., sits the massive headquarters of a spy agency few know exists. The [National Geospatial-Intelligence Agency, or] NGA remains by far the most shadowy member of the Big Five spy agencies, which include the CIA and the National Security Agency. Despite its lack of name recognition, the NGA's headquarters is the third-largest building in the Washington metropolitan area. The NGA is to pictures what the NSA is to voices. Its principal function is to analyze the billions of images and miles of video captured by drones ... and spy satellites. The agency has never been involved in domestic spy scandals. However, there's reason to believe that this will change. In March 2016, the Pentagon released the results of an investigation initiated by the Department of Defense's Office of Inspector General to examine military spy drones in the United States. The report ... revealed that the Pentagon used unarmed surveillance drones over American soil. The investigation also quoted from an Air Force law review article pointing out the growing concern that technology designed to spy on enemies abroad may soon be turned around to spy on citizens at home. In 2016, unbeknownst to many city officials, police in Baltimore began conducting persistent aerial surveillance using a system developed for military use in Iraq. Few civilians have any idea how advanced these military eye-in-the-sky drones have become.