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John Keel, who died on July 3 aged 79, was a prominent American ufologist (an observer and chronicler of UFOs - unidentified flying objects) and the author of The Mothman Prophecies (1975), a book about paranormal phenomena which was made into a successful film starring Richard Gere.

One of ufology's most widely-read and influential authors, Keel became an original and controversial researcher, and is credited with coining the term MIB (Men In Black), sinister and threatening entities who assume human form to confront ufologists and UFO witnesses.

Of particular importance was Keel's analysis of patterns. His work on "windows" (specific hot spots of combined phenomenal appearances), "waves" (cyclic appearances of the phenomena) and the "Wednesday phenomenon" (the theory that a disproportionate number of UFO events occur on that day of the week) influenced scholars and followers of the genre alike.

In his much-acclaimed second book, UFOs: Operation Trojan Horse (1970), Keel suggested that many aspects of modern UFO reports, including humanoid encounters, often paralleled ancient folklore and religious visions, and directly linked UFOs with elemental phenomena.

"Ufology is just another name for demonology," Keel explained, and claimed that he did not consider himself a "ufologist" but a "demonologist"; as an early admirer of Charles Fort (1874-1932) he actually preferred to be called a Fortean, which covers a wide range of paranormal subjects.

The Mothman Prophecies was Keel's account of his investigation into sightings in West Virginia of a huge, winged creature called the Mothman. Loosely adapted into a 2002 film starring Gere and Alan Bates, who played two parts of Keel's personality, the book explored the problems facing a UFO investigator when he becomes personally caught up in the unfolding of paranormal events.

The Mothman - so named by an excitable newspaper subeditor - was reportedly first encountered in November 1966, and again, repeatedly, the following year. Sightings dwindled following the collapse of a nearby bridge during the evening rush-hour in December 1967, in which 45 people were killed; the red-eyed apparition is popularly believed to presage or even cause disasters.

As well as producing novels such as The Flying Finger of Fate (1966), Keel began writing articles for Flying Saucer Review, a British-based publication which claims to number the Duke of Edinburgh among its readers.

Also in 1966, Keel became a full-time investigator of assorted paranormal phenomena, and for the next four years interviewed thousands of people in more than 20 American states. At first he sought to explain UFOs as extraterrestrial visitations. But a year into his investigations, Keel realised that this hypothesis was untenable.

"I abandoned the extraterrestrial hypothesis in 1967, when my own field investigations disclosed an astonishing overlap between psychic phenomena and UFOs," Keel wrote. "The objects and apparitions do not necessarily originate on another planet and may not even exist as permanent constructions of matter. It is more likely that we see what we want to see and interpret such visions according to our contemporary beliefs."

After investigating incidents of paranormal telephony - spirits supposedly communicating electronically - Keel found his phone calls being mysteriously re-routed to another number, one digit different to his own. Oddly, the person answering claimed also to be called John Keel; odder still, the voice of the doppelgänger sounded remarkably similar to Keel's own.

John Alva Keel was born Alva John Kiehle on March 25 1930 at Hornell, New York State, the son of a small-time bandleader. His parents split up when he was very young, and he was brought up by his grandparents until his mother remarried. Fascinated by magic from an early age (his friends called him Houdini), he had his first story published in a magicians' magazine when he was 12.

Having become a columnist with a local newspaper when he was 14, Keel started to make a living writing pulp articles ("Are You A Repressed Sex Fiend?" was a typical title), became a scriptwriter for radio and television, and a freelance contributor to newspapers.

Drafted into the US Army during the Korean War, Keel served in Frankfurt on the staff of the American Forces' Network. After leaving the military he worked as a foreign radio correspondent in Paris, Berlin, Rome and Egypt, but when he was 24 he resigned and blew his savings on a four-year odyssey around the Middle East and south-east Asia.

The result was his first published book, Jadoo (1957), an account of his journey to India to investigate the activities of fakirs and holy men who perform the Indian rope trick and survive being buried alive. He also recounted tracking a yeti (the Abominable Snowman).

Keel first identified the so-called "Men In Black" in an article for Saga magazine in 1967 headed "UFO Agents of Terror". He described sinister figures of gaunt, evil aspect, often with oriental or Hispanic features, a phenomenon he noticed again in January 1969 during President Nixon's first inauguration.

"I was very interested to notice three men in black suits looking very much like our classical men-in-black sitting together a few rows from the front, right behind Nixon when be gave his inaugural address," Keel wrote. "Every time the television cameras shot Nixon from a particular angle, I could see these three men. They seemed out of place. Of course they could have been ambassadors from Vietnam or something.

"I wondered afterward if my imagination had been running away from me. I got a hold of all the magazines I could find with pictures of the inauguration; and I went over them with a magnifying glass; but I could not find those three guys. Yet I had seen them very clearly on television."

In Our Haunted Planet (1971), Keel coined the term "ultraterrestrials" to describe UFO occupants. He discussed the seldom-considered possibility that the alien "visitors" to Earth are not visitors at all, but an advanced terrestrial civilisation, consisting of shape-changing phenomena from another order of existence, which may or may not be human.

Keel, considered a creative and proactive investigator of UFOs, believed ultraterrestrials and their minions could manifest themselves as monsters, space people, ghosts or other paranormal entities. He reasoned there were two kinds: "good and bad guys".