U.S. spy agencies have concluded that al-Qaeda faces “substantial” challenges in devising an effective way to attack drones, according to the top-secret report disclosed by Snowden. Still, U.S. officials and aviation experts acknowledge that unmanned aircraft have a weak spot: the satellite links and remote controls that enable pilots to fly them from thousands of miles away.

In July 2010, a U.S. spy agency intercepted electronic communications indicating that senior al-Qaeda leaders had distributed a “strategy guide” to operatives around the world advising them how “to anticipate and defeat” unmanned aircraft. The Defense Intelligence Agency (DIA) reported that al-Qaeda was sponsoring simultaneous research projects to develop jammers to interfere with GPS signals and infrared tags that drone operators rely on to pinpoint missile targets.

Other projects in the works included the development of observation balloons and small radio-controlled aircraft, or hobby planes, which insurgents apparently saw as having potential for monitoring the flight patterns of U.S. drones, according to the report.