From outside, the National Archives of England and Wales seems an unlikely setting for a whodunnit. Set back from the bank of the Thames in Kew, south-west London, its architecture is a combination of the forbidding and the friendly. The original 1974 Public Record Office building, a fortress-like concrete structure resembling a Modernist���s attempt to build a beehive, is now dominated by a big new wing, built in 21st-century university-annexe-style sandstone.

Inside, there is slightly more of an air of mystery. The labyrinthine building contains 110 miles of shelving, holding the written record of British and Imperial governance stretching back to the Domesday Book (an original copy of which also lives there). Surrounding those long, dark corridors are the administrative organs: the rooms occupied by curators, conservators and researchers who keep existing records safe and sift through the constant flood of new bureaucratic material coming in from Whitehall, cataloguing it for the Archives��� ���customers���.

Those customers, known as readers, inhabit only the outside layers, the thin, public skin of the institution. Here, in the reading rooms, you will see retired accountants rubbing shoulders with celebrity historians: Sir Max Hastings, consulting original sources for his latest book, sits at the same table as Mr Smith from Liverpool, sifting merchant navy records for clues about his father���s war. There are historians constructing books, postgraduates eking out theses, paid researchers working for companies or foreign governments, professional genealogists delving for missing branches of family trees, even the occasional journalist seeking exclusives that escaped his colleagues 30 years ago.

The quiet of the reading rooms belies a seething mass of jealousies and agendas. Each historian tries to outrun the next, each researcher jealously guards her discoveries, each journalist hides his scoop.

Into this world, sometime around the turn of the millennium, came someone carrying at least 29 counterfeit documents. That person was bent on rewriting the past and on changing the way in which Britain regards its own reputation and that of its greatest wartime leader, Winston Churchill. The breach of security alarmed every national archive in the world, institutions which concentrate on catching people trying to smuggle documents out rather than in. And so began an investigation based on that deceptively simple question: whodunnit?

The e-mail I received from a colleague in June 2005 should not have come as a surprise. He was asking me to investigate allegations that a British intelligence agent had, on the orders of Winston Churchill, murdered Heinrich Himmler, the head of the Nazi SS, in 1945.

The claims, which besmirched the reputation of Britain and its wartime prime minister, were made in a book called Himmler���s Secret War by Martin Allen. It was his third book about Britain���s relationship with the Nazis. Each challenged the standard version of events. In the case of the latest, it appeared to refute the accepted account of Himmler���s death: that the enforcer of the Holocaust committed suicide hours after his capture by British troops on May 23 1945, 15 days after the end of the war in Europe.This claim should not have come as a surprise to me because a fortnight earlier I had heard Allen discussing the ramifications of his claims on Radio 4���s Today programme. I paid little attention because, frankly, people are always making outlandish claims about the Nazis. What I hadn���t realised, distracted by making breakfast for my children, was that these claims ought properly to have been called ���discoveries���. The e-mail I was now reading emphasised that Allen, in revealing the brutal behaviour of the British Establishment 60 years before, cited documents he had found in the National Archives.

I had a great deal of experience there, having ferreted around for undiscovered nuggets of history for the best part of a decade. For me, as for anyone who works within its quiet walls, cloaked in the faint but everpresent smell of dust, it is the fount of authenticity. Regular users are, by definition, somewhat attached to the past, so most still call it the PRO, short for the Public Record Office, the name the institution held for 167 years. And the PRO is the memory bank of England. If you seek the whos, whats, whens and wheres of British history, then look in old newspapers, encyclopedias or in children���s textbooks. For the hows and whys, consult the PRO: motivations, fears, embarrassments, ambitions. They are all there, waiting to be found in that endless shelving.

Of course, some of those secret files might contain lies and deceptions. But they belonged there: official lying, deception in the name of the Crown. So if the documents Allen cited were in the PRO, surely that was proof that British intelligence had murdered Himmler?

My colleague���s e-mail had three attachments. A friend of his had been so astonished by the claims in Allen���s book that he visited Kew to photograph the three documents that were key to the murder claims. My doubts about them were instant. First, the language seemed too modern. You get used to the tone of official documents from different eras. Every evolution of mandarinese has its own buzzwords. The words of these documents, two letters and an enciphered telex, would have been more likely to have come from a Frederick Forsyth character than from serious-minded Foreign Office officials such as John Wheeler-Bennett and Robert Bruce-Lockhart, the supposed authors. Did such men really discuss the need to ���eliminate��� an enemy? Would Wheeler-Bennett, author of a dozen highly regarded biographies and histories, commit the ugly tautology of saying they should ���expedite such an act with some haste���?

Even more suspect was the third document, a letter to the head of the Special Operations Executive written by Brendan Bracken. Would as fastidious a literary stylist as Bracken, Churchill���s political henchman and Minister of Information (and the founding father of the modern Financial Times, whose statue stands in the newspaper���s London offices to this day), really have written that it would have ���devastating repercussions��� if the supposed murder of Himmler came to the attention of Britain���s allies?

I did not know enough about the history of that period to judge whether senior British intelligence staff would have considered it morally, politically or strategically justified to kill Himmler in cold blood. But I was pretty sure that if they had, they would not have left casual written records of their deeds. One document even mentioned the name of the alleged assassin ��� ���Mr Ingrams��� (this was supposed to be Leonard Ingrams, in real life the father of Richard, the founding editor of Private Eye).

The following day I went to Kew and looked at the original documents. They appeared authentic: they seemed to be attached to the string treasury tags used to bind the top left-hand corner of the file; the paper looked old; the typefaces did not stand out from the dozens of other sheets of paper in the files. But on closer inspection of the Bracken letter, I saw what I was sure were pencil marks beneath the signature. Taken together, I had enough doubts to pursue the next step, which was to try to have them professionally tested. I was lucky that the newspaper I worked for then, The Daily Telegraph, trusted my judgment, even though I had just a hunch to go on and the forensics would cost at least ��1,000.

I asked the PRO���s permission to have the documents examined by an expert, not really expecting them to agree. But the senior officials to whom I showed the documents shared my doubts. A police contact recommended Dr Audrey Giles, a former head of Scotland Yard���s Questioned Documents Unit. She was already known to the PRO, having authenticated the ���black��� diaries of Roger Casement, hanged for treason in 1916.

The Himmler documents, along with other files containing confirmed examples of the handwriting of the three men principally implicated, were taken by PRO officials to Giles���s laboratory in Buckinghamshire. A few days later, she confirmed that the documents were certainly forgeries.

Among other findings, she confirmed the presence of the pencil trace lines under Bracken���s signature. Although each paper had looked as though it was attached to the treasury tag that held all the other papers together, under closer examination it became apparent that all three had a tiny tear between the punch hole in the top left corner and the edge of the sheet, allowing them to be slipped around the string of the tag without having to unpick the whole file.

Giles also revealed a clinching piece of evidence: the Ministry of Information address printed on Bracken���s 1945 letter had been produced using a laser printer. Giles reported that she had found two further Bracken letters in the file that were also certain forgeries. This was the first hint that other elements of Martin Allen���s book were based on bogus evidence.

Textual analysis confirmed that something was wrong with the papers, and this was explained to police by the PRO���s own experts: they pointed out that the counterfeit documents contain errors, breaches of protocol and etiquette their supposed authors would not have committed. To take one example, in a letter to the Foreign Office dated October 1943, Victor Mallet (grandfather of the FT���s Asia editor), refers to himself as His Majesty���s Ambassador in Sweden. But, at the time, Mallet was not an ambassador; he was ���Envoy Extraordinary and Minister Plenipotentiary���. Such a gaffe is highly improbable.

One PRO expert pointed out to the police that another common thread of the forgeries was that ���they were found amongst files that bear little or no relation to their own contents���.

She added that the inconsistencies on the papers ���would lead any serious historian to question their veracity���.

With the exception of a single forged document found in the US National Archives in Washington in 1985, and apparently aimed at proving the existence of UFOs, nothing like this had happened before.

Armed with these results, the next day I interviewed Allen by phone. He said he was astonished and ���absolutely devastated��� by the findings. The Daily Telegraph led with the story the next day, with a separate account of Allen���s denial of any involvement in the forgeries.

On the Monday, as historians and researchers reacted with outrage to what had happened, I went back to the PRO and looked at more documents cited in the book. At least one other paper did not appear to be properly connected to the tag.

A few days later, a German academic, Ernst Haiger, told me about his own suspicions over other PRO documents cited in an earlier Allen book. I went to see Allen at his home ��� he had stopped answering his phone ��� eager to hear his theory of what had happened. When I eventually approached him outside his small, modern, detached house in the south-west of England, I found a diminutive, nervous-looking middle-aged man. He was not expecting me and he blanked me, refusing to answer questions and referring my inquiries to his agent in London.

Back at the PRO, I found more documents in other files referred to in both the Himmler book and Allen���s second book, The Hitler/Hess Deception, which looked suspicious. By now, the PRO had commissioned its own testing and confirmed Giles���s results. My searches, Ernst Haiger���s evidence and the PRO���s efforts eventually led to more than a dozen documents being identified as suspicious and submitted to Home Office specialists for examination. When they too were declared forgeries, the PRO called the police in.

i moved on to other stories, confident that the police would soon issue a report, maybe even arrest somebody. Allen was obviously a suspect, but he was a man who seemed to have no great affinity with technology ��� he did not even have an e-mail account ��� so I doubted whether he would have been able to produce relatively sophisticated forgeries.

Things went quiet. I was interviewed by Det Insp Andy Perrott, a local CID man but with experience in the Fraud Squad. He seemed slightly surprised to be investigating such a case, but also fascinated with the details.

Months wore on, as often happens with police investigations of complex frauds. Then, on one bizarre day in 2007, I learned that although suspects had been interviewed, one even arrested, there were to be no charges. The investigation was at an end.

Surprised, I rang the Crown Prosecution Service, and was read a short statement saying that it had been decided that it was ���not in the public interest��� to prosecute the only suspect questioned by police. It did not name the suspect or give a reason. This was a surprising development because it seemed to suggest that there was no public interest in protecting the integrity of the National Archives. Once I had made a few phone calls and got more background material from the PRO under the Freedom of Information Act, I had a clearer picture of what had happened. After the furore surrounding his latest book, Allen���s health problems prevented the police questioning him for nine months. When they did, he told them that he was wholly innocent. Allen said he was an assiduous researcher and had read every file that he could find that might touch on his subjects.

Inevitably, therefore, he was the one who had come across these forgeries, although he had not suspected that any document he used was counterfeit. He thought it might all be a conspiracy. In fact, in the addendum to the American edition of the book (which acknowledged the fact that the ���Himmler murder��� papers were forged, but nonetheless repeated the allegations they contained), Allen posited his own theory. At some time after he saw the documents, he suggested, they had been removed and replaced with exact replicas, clumsily forged to cast doubt on his discoveries. In the absence of any other public statement by him, this is the only explanation that Allen is known to have put forward.

Perhaps this was the true scenario: Allen, a self-styled ���eminent historian���, stumbled on the documents during painstaking research that took him to files left untouched by other historians; then, after his books were published and unknown forces read about his discoveries, the only way they (presumably modern British intelligence agents) could discredit him was to substitute forgeries in the files for the genuine ���smoking gun��� documents.

But the police investigation, relying on Forensic Science Service tests, finally revealed that this had not just happened a few times. In all, there were 29 forged documents, each typed on one of only four typewriters. They were placed in 12 separate files, and cited at least once in one or more of Allen���s three books. In fact, according to the experts at the Archives, documents now shown to be forgeries supported controversial arguments central to each of Allen���s books: in Hidden Agenda, five documents now known to be forged helped justify his claim that the Duke of Windsor betrayed military secrets to Hitler; in The Hitler/Hess Deception, 13 bogus papers supported Allen���s contention that, in 1941, British intelligence used members of the Royal Family to fool the Nazis into thinking Britain was on the verge of a pro-German putsch; in Himmler���s Secret War, 22 counterfeit papers also underpinned the book���s core claims that British intelligence played mindgames with Himmler to encourage him to betray Hitler from 1943 onwards, and that ultimately they murdered the SS chief.

None of these forged documents was cited in any other works of history. Allen disputes this, but has not provided any citations.

Computerised PRO records beginning in 1994 prove who has seen the 12 files between then and the publication of the books. They prove that only two people had seen more than three of the compromised files since they had been transferred from their respective government departments ��� the Cabinet Office, the Foreign Office and MI6. Those two people were Martin Allen and his wife Jean (whom police ruled out as a suspect).

This would, of course, have been the case if Allen had been victim of the kind of conspiracy he claimed. But evidence volunteered to the inquiry by the publisher of one of Allen���s books threw doubt on that idea. The publisher had asked Allen to show him copies of four ���smoking gun��� documents and, months before publication, the author had produced photocopies. They were identical to the document sent to the forensic experts, even down to the forged handwriting style. This strongly suggested that, at least in these cases, Allen���s theory of a conspiracy could not be true.

so what happens now? Last December, in response to questions from Norman Baker MP, the Solicitor-General said that the police investigation, guided by the opinion of a senior barrister, had produced ���sufficient evidence for a realistic prospect of conviction��� on charges of forgery, using a forged document and criminal damage against Allen. But it had been decided that it was not in the public interest to proceed. In reaching that decision, ���matters relating to Mr Allen���s health and the surrounding circumstances were significant in deciding that a prosecution was not in the public interest���. This statement puts the author in an invidious position, his books fatally undermined as works of serious history and him unable to clear his name.

But many historians and researchers feel there is also the question of the reputation of the National Archives. Is it not in the public interest for that institution to be ���cleared���? Of course, some people might feel that it doesn���t really matter. It was all a long time ago. But others disagree. They are not orthodox historians or politicians or family members of men whose reputations are stained by the forgeries, but people who believe that the version of history that almost all historians accept should definitely be rewritten.

Allen���s books and the discovery of the forged documents have been much debated on websites and message boards frequented by revisionist historians, people whose common theme is that the orthodox view of the Nazis or the Holocaust is biased against Hitler and in favour of his victims. To these people, the failure to prosecute anyone for the forgeries is grist to the mill of their conspiracy theories.

What light could Allen shed on the affair? In pre-dawn drizzle I headed towards the West Country, where he had lived since the storm broke over his Himmler book. At breakfast time on that March morning, Allen���s wife answered the door. She told me her husband was not in. He was very ill and neither of them was interested in answering any questions. ���This matter is closed. It has been closed for more than a year,��� she said before closing the door. The next day an e-mail came from Allen���s solicitor. It repeated his denial of any knowledge of how the forgeries had appeared and demanded that he be left alone. It also rejected suggestions that the revelations undermined Allen���s work and denied any intention to distort history.

As I drove back to London I passed Stonehenge, rainswept and glistening in the sunlight creeping through a fractured blanket of cloud, its presence a reminder that the further we get from events of the past, the more we cling to scraps of information to explain the mysterious behaviour of our predecessors. And as the witnesses to this behaviour die, we rely on historians to supply a balanced account.

In 2005 there was a telling interpretation of my original discovery of the forgeries in the PRO. It came from Gert Sudholt, Allen���s publisher in Germany, stepson of the former deputy head of Goebbels���s press propaganda machine and a man who has served six months in a German prison for publishing neo-Nazi material.

He said: ���The most pressing questions for us now are, if the documents were forged, where did they come from, and who put them there, and why were they in the National Archive? Can we trust anything in the National Archive at all for the past 500 years? Can we indeed trust anything in any national archive? This could lead to the review of contemporary history and turn it on its head.���

in the convoluted world of revisionists, even clear evidence that the documents Allen relied on becomes (in the absence of proof of who faked them) in itself evidence that other historical episodes should be questioned and revised. Maybe that should have been the real ���public interest��� question for the Crown Prosecution Service. Would it be in the public interest of Britain, or any other country, for the history of the last 60 years to be ���turned on its head���? If that happened, we might be faced with new interpretations of the past, new rewrites of a story we thought we knew, based on documents we thought we could trust, conducted by people we can���t.

And wouldn���t that pose far more serious questions about our history and our future than the deceptively simple one: whodunnit?

Ben Fenton is the FT���s media correspondent