Early reports that Blatty based his 1971 novel on a story of demonic possession that he learned of while a student at Georgetown University in 1949 were confirmed by his own book-length account of the creative process behind the story, William Peter Blatty On The Exorcist From Novel To Film (1974). He writes that he was a 20-year-old English Literature major when he read an article in the Washington Post (20 August 1949) by Bill Brinkley. It told of a 14-year-old boy, of Mount Rainier, Maryland, who had been freed from a tormenting devil by a Catholic priest earlier that year.

Blatty began writing The Exorcist in 1969, drawing upon the material he had discovered some 20 years earlier, and finished during the summer of 1971.

In his book, he tells of writing to the priest who conducted the actual 1949 exorcism and learning of a diary kept by an assisting priest, recording the daily events of the on-going exorcism - see diary panel. The exorcist refused Blatty’s request to see the diary so, to ease the priest’s anxiety, Blatty changed his central character from a 14-year-old boy to a 12-year-old girl. Eventually, Blatty gained access to one of a number of copies of the diary and based much of his book and movie on its contents.

Rather than present the whole confused legend and then set about refuting it, I’ll here set out the sequence of events as far as I have been able to establish them. Rob Doe - as I prefer to call him - was born on 1 June 1935, making him around 13 when the exorcisms were conducted. The Doe family were German Lutherans and lived with the boy’s grandmother who barely spoke English.

We know that, in January 1949, members of the family - probably led by Rob’s mother and grandmother - began experimenting with a ouija board. The disturbances began around 18 January; scratching noises came from the walls, the boy’s bed would shake violently and objects (such as fruit and pictures) would jump to the floor in the boy’s presence. Rob - if he was suspected at all - claimed to be possessed by an "invisible entity". It is significant (from the diary entries) that Mrs Doe suspected that there was a connection between the strange events and the death of ‘Aunt Tillie’, a spiritualist who had introduced the family to the ouija board. At various points throughout this ordeal, Mrs Doe attempted to communicate with Aunt Tillie, apparently alternating the beliefs that the problems with her son were either the work of the Devil or their departed relative.

The family agreed to let Rob spend a night (17 February) at the home of their local priest - the Reverend Luther Miles Schulze, pastor of St Stephen’s Evangelical Lutheran Church in Washington DC - so he could observe the boy. Despite his scepticism, Fr Schulze told of hearing vibrating sounds from the boy’s bed and scratching sounds on the wall. During the course of the night, he allegedly witnessed a heavy armchair (in which the boy sat) tip over seemingly on its own and a pallet of blankets on which the boy lay inexplicably move around the room. He called in the family doctor, who prescribed phenobarbital for the whole family.

My investigation led me to conclude that the mother made the initial contacts - she took the kid to the rectory and later to the Georgetown University Hospital - and not Father Hughes as the legend has it. "She’s the one who gave Father Hughes all the information," said Father Bober to me later. A source verified for me that Rob Doe was admitted to Georgetown University Hospital under his real name on the morning of Monday, 28 February 1949 and released three days later.

Despite what is written in Possessed, there is no evidence that Father Hughes was ever suffering theological doubts or confusion at all about this situation - (see exorcists panel). In fact, there is no written record of the alleged exorcism attempt at Georgetown University Hospital (except for the frustrated attempt to baptise Rob, mentioned in the diary).

So what really happened? I have shown that Father Walter Halloran - the sole living eyewitness to the St Louis exorcism attempts - maintains that he did not witness any supernatural behaviour by Rob Doe: no strange foreign languages (other than mimicked Latin), no changes in tone of voice, no prodigious strength, no excessive vomiting or urinating. To top it off, he is uncertain about the nature of the markings on the boy’s body. The credibility of the important diary has been called into question.

Personally, I do not believe Rob Doe was possessed. There is no question there was something wrong with Rob Doe prior to January 1949, something that modern psychiatry might have addressed better.

There is simply too much evidence indicating that, as a boy, he had serious emotional problems stemming from his home life. There is not one shred of hard evidence to support the notion of demonic possession. The facts show that he was a spoiled and disturbed only child with a very overprotective mother and a non-responsive father (sources close to the family told me he did not believe the boy was possessed).

To me his behaviour was indicative of a lonely youth who desperately wanted out of Bladensburg Junior High School at any cost. Throwing tantrums was the answer. He was rewarded by having a collection of priests (who had no previous exorcism experience) doting over him as he lay strapped to a bed. His response was that of any normal child - he reacted with rage; he wanted out.

Each of the parties involved in this case approached it from its own frame of reference. To psychiatrists, Rob suffered from mental illness; to priests this was a case of demonic possession; to writers and producers this was a great story to exploit for profit. Those involved saw what they were trained to see. Each purported to look at the facts but just the opposite was true; in actuality they manipulated the facts and emphasised information that fitted their own agendas.

The story of Rob Doe went on to spawn movies, books, and videos, and influenced hundreds of ‘copycat’ cases around the world, leading to exorcism-styled assaults, mutilations, and even deaths. Consequences of this scale make it imperative, in paranormal investigation, for close scrutiny of the initial accounts and providers of information. In this instance, the primary sources muddled the picture by embellishing the story when facts were uncertain.

This version of the article has been edited for the FT web site.