In the U.S. they would be characters who appear after a UFO incident report in order to convince whistleblowers not to talk about it. They wear black suits and were initially of Inuit characteristics, but later described them as Caucasian. They would be associated with the Air Force (their beginnings coincide with the Blue Book project, in the mid-20th century) or some government agency. They inspired characters from novels, comics, movies and television series. From English Men in Black.
