During the 1950s, after studying the way trains in Britain pick up mail bags by the side of the tracks, Fulton developed the Fulton surface-to-air recovery system, also called the "Skyhook" for the Central Intelligence Agency (CIA), the United States Navy, and the United States Air Force. It was a system that was used to pick up people from the ground with an aircraft. It was used by the United States Air Force until 1996. A sister invention for Navy frogmen was called "Seasled".