Did an alien spacecraft crash in New Mexico in 1947; were four extra-terrestrials, one perhaps still alive, removed from the site; and has the whole episode been the subject of the most intense information blackout by the US military and security services ever since?

Stanton T Friedman is convinced that the answer to all those questions is "yes", but unlike most ufologists he has based his conclusion not on a quasi-religious belief that flying saucers are real, but on years of detailed research. Unlike many of his co-believers, he also has the credentials to give some weight to his findings. For when he is not out pursuing flying saucers, Mr Friedman is a nuclear physicist.

Last week "the nuclear physicist lecturer Stanton T Friedman", as he describes himself on his many publications, gave a lecture at the Royal Geographical Society in Kensington Gore. While anyone listening might not be guaranteed to come away believing in visitors from outer space, they can hardly fail to be impressed by his perseverance towards the goals of extra-terrestrial life, witnesses and the pursuit of freedom of information.

Mr Friedman, a short and round man of 60 with heavily whiskered jowls and a penetrating glare, is the author of such academic papers as Preliminary shield design for nuclear electric space power plants and Measurements of secondary gamma-rays from tungsten, stainless steel and borated steel. He has also written Flying Saucers and Physics ("explaining why UFOs do not violate the laws of physics") and given his lecture Flying Saucers Are Real in hundreds of colleges and professional institutions.

He has been researching flying saucers since 1958, with particular interest in reports of the crash of an alien spacecraft at Corona, near Roswell, New Mexico, in 1947.

There is no doubt that something crashed. The front page of the Chicago Daily News of 8 July 1947, beneath its main story, "American All-Stars Win 2-1", carries a second lead headed: "Army Finds Air Saucer on Ranch in New Mexico". Later editions, however, downgrade the story with a statement from the military: "General believes it is radar weather balloon".

The US Air Force certainly investigated the crash and took away wreckage. Mr Friedman has tracked down enough eye-witnesses and even participants to confirm that something was there. There was talk, too, of body bags.

Fuelled by consistent refusals by government departments to declassify any of the information connected with the case - or, indeed, with almost anything connected with flying saucer research - Roswell has become the holy grail of ufologists. And Mr Friedman, a passionate and persistent collector of data, has been patiently pursuing the quest for evidence.

Interviews with those involved, or their relatives, produce repeated references to alien bodies and strange thin pieces of metal that bounce back into shape when distorted. Demands for silence supported by threats from the security services add to the mystery, and there is even a report of a local undertaker being asked about the smallest size of body bags he had in stock.

Through the courts and the Freedom of Information Act, Mr Friedman has succeeded in obtaining a few documents relating to the case, though many are heavily censored with page after page blacked out until only a few bland words remain. He might, to a sceptical ear, appear to read a great deal into those blacked-out passages.

There is no doubt that something is being covered up, but whether it is, in Mr Friedman's words, "a cosmic Watergate" and "the biggest story of the millennium" must await declassification of another million or so documents. Which may, after all, reveal the location of the bodies of crashed aliens, and confirm that we are being watched closely by superior beings who think us worth monitoring.

It may not be long before we have an answer. This month, the US General Accounting Office is expected to publish its investigation on the secrecy surrounding air force documents relating to the Roswell incident. A report in the Washington Post has already indicated that the GAO has reached the conclusion that the whole truth has not been told.

Last December, in Stanton-by-Dale, Derbyshire, 30 people reported a UFO sighting to the local police. The official explanation was that an electricity cable had exploded in a shower of blue sparks. Another conjecture, however, might be that it was a UFO, come down for a closer look at Mr Friedman, only they picked the wrong Stanton.