UFO sightings and alleged contacts with "extraterrestrials" have often been accompanied by the shady and mysterious "Men in Black". We all know about them, how they are supposed to pose as military officers, UFO researchers or whatever and make threats and/or demand evidence to be turned over to them.

UFO sightings and alleged contacts with "extraterrestrials" have often been accompanied by the shady and mysterious "Men in Black". We all know about them, how they are supposed to pose as military officers, UFO researchers or whatever and make threats and/or demand evidence to be turned over to them.
Most of it is modern folklore that can be traced to They Knew Too Much About Flying Saucers, a ’50s book which turned out to be an enormous hoax, but there have been a few interesting cases, including at least one investigated by law enforcement agencies.
Men in Black seem to be a world phenomenon, since they have been reported from the United States, Spain, France, Italy, Chile, Brazil, Mexico etc. Surprisingly apart from a couple of very dubious cases I have been unable to find any similar case from Britain or Ireland.
Does anyone know more?
Thanks!