John Keel (1930-2009) wrote several classics of Fortean literature* dealing with UFOs, cryptozoology, poltergeists and various other anomalous data. He came from a newspaper and radio background and brought that charismatic energy to his research, often playing a role in the events he described. His approach to these subjects changed drastically over time. Originally Keel backed the nuts-and-bolts theory that says UFOnauts are natural, fleshly denizens of some other solar system [Zeta Reticuli?] -- popularly termed the Extra-Terrestrial Hypothesis. Eventually -- like his contemporary, French researcher Jacques Vallee -- Keel began to notice that other sorts of unexplained phenomena occurred in tandem with UFO flaps. For example, cases where poltergeist-like activity occurred in the homes of UFO witnesses preceding and/or following the UFO event. This approach lead to the idea that perhaps UFOs and abduction experiences, bigfoot and dog-man sightings -- that all these things are modern manifestations of the same principle that revealed itself as ghosts, fairies, angels, demons and gods in the past [also supported by Vallee in Passport to Magonia].
* The Mothman Prophecies, Our Haunted Planet, Disneyland of the Gods