Everything seemed to be proceeding smoothly as the Dakota transport aircraft, bearing the insignia of the 110 Squadron of the Royal Air Force, took off from Kai Tak airport on the morning of September 25, 1946.

The air-accident investigation report indicates that the aircraft quickly climbed to between 700 feet and 800 feet before the pilots inexplicably lost control of the plane and crashed some 3km from the airfield.

Of the five crew members and 14 passengers aboard the flight bound for Singapore, none survived.

The report concluded that the crash had been caused by turbulence from the surrounding hills, which created a crosswind of as much as 25 knots at 30 degrees to the flight path. It also criticised the pilots for failing to realise the danger posed by the turbulence and terrain ahead.

War veteran Arthur Lane, however, does not believe that explanation. It is far too convenient, he says, that the main British investigator into crimes committed by the Japanese during the second world war was aboard the aircraft. What's more, his probing was building a solid case that could have seen the Japanese emperor held accountable for some of the worst atrocities carried out by his troops in the early decades of the last century.

Lane claims Major Cyril Wild was killed to preserve the imperial lineage in Japan and halt the spread of communism across Asia.

As part of a wider cover-up, Lane believes, the United States military gained access to files containing information on the chemical, biological and bacteriological warfare which the Japanese military had tested on civilians in China and other parts of Asia.

BORN IN STOCKPORT, northeast England, where he lives today, 94-year-old Lane joined the Manchester Regiment as a drummer and bugler in 1936 and saw service in Egypt and Palestine before arriving in Singapore in late 1938. After war broke out in the Far East, Lane survived air raids from Japanese bombers and, after ground troops landed on Singapore island, came under mortar attack.

When Singapore fell to the Japanese on February 15, 1942, Lane and Wild were both among the 80,000 British, Australian and Indian troops who became prisoners of war .

Wild was unlike the vast majority of men captured at the surrender of the island fortress, which Britain considered to be its "Gibraltar of the Far East". He had already spent many years in the Asia-Pacific region, as an executive of Sun Oil, a major Japanese petroleum company. Having lived in Japan, he spoke the language, was familiar with the national psyche, had seen the rise of militarism and nationalism and had belonged to a Christian group in Tokyo, building a network of Japanese friends.

He was also a brave man. Several photos survive of the delegation of officers from the British garrison in Singapore marching towards the Japanese lines to surrender, headed by Lieutenant General Arthur Percival and a Japanese officer. Alongside Percival was Wild, carrying a white flag. Shortly after the dispirited party had set out, Wild threw his flag of surrender to the ground and said, "This is stupid. Let's go back."

Overruled and ordered to continue, Wild remained defiant during his 3½ years in captivity, initially in Singapore and later as a slave labourer on the Japanese Army's infamous Burma railway.

It was in Burma that Lane learned more about the officer and the secret record-keeping he had conducted in anticipa-tion of the time justice could be served.

"Although I was not close to Major Wild, I knew him and the task that he had set himself while he was a prisoner of war," Lane tells Post Magazine.

"Prisoners were asked to pass on to Wild's 'office' - for want of a better name - all information concerning the ill-treatment of British and Australian personnel.

"I was a bugler … and, as such, I had to attend many funerals in prison camps along the route of the railway," Lane says. "I was able to supply Major Wild with the names of any men who had not died of natural causes. Those who had been killed."

During his years in captivity, Wild built up - at huge personal risk - rudimentary dossiers on dozens of Japanese whose actions made them liable to face trial for war crimes. He hid his information in a vestment case that belonged to the padre and which Wild carefully buried close to the camps where he was interned.

"Throughout the time I knew him, he was a very strong, courageous, determined soldier," Lane says.

Wild had compiled statements and lists of more than 1,000 criminal acts, including that which sealed the fate of the 7,000 British and Australian troops who made up F Force. The troops had set out from Singapore's Changi prisoner-of-war camp for what the Japanese termed "health camps", which, it was claimed, would offer medical and sports facilities and a better climate. The Japanese assured Wild that the journey would entail no marching and that the men would not be put to forced labour.

After a five-day train and truck journey, the men were told to start marching. Prisoners already weakened by disease and injury received no food or water for the last 24 hours of the trip. By the time they reached their destination, some 300km further on and after the monsoon had broken, they had been forced to abandon virtually all their kit and medical supplies.

Despite outbreaks of cholera and dysentery, compounded by malaria and malnutrition, on arrival the men were immediately put to work on a 50km stretch of railway. Beatings were constant and the sick were forced to the construction sites at the point of a bayonet.

According to the report prepared post-war by Wild for the trials of Japanese officers, "The attitude of the Japanese towards the sick was a mixture of callous indifference and active spite for, by their sickness, they were regarded as impeding the Japanese war effort."

Of the 7,000 men who left Changi, only 3,000 returned two months later. Of those, just 125 were passed as fit for light duties by Japanese medical officers six weeks later.

Against the odds, both Lane and Wild survived the war. At a small gathering in Rangoon after their liberation, the major "solemnly promised that he would pursue to the ends of the Earth those Japanese responsible for the murder and ill-treatment of Allied prisoners of war", Lane says.

After a brief period of leave in Britain, Wild was promoted to the rank of colonel and given the task of identifying, locating and trying Japanese for war crimes. By all accounts, Wild set about his task with gusto and was looking into a number of atrocities.

These included the December 1941 shooting, bayoneting and beheading of 53 prisoners of war at Eucliff villa, in Repulse Bay. A couple of days later, 96 wounded soldiers were killed by rampaging Japanese troops in St Stephen's College, which had been turned into a casualty station.

Another incident that Wild investigated was the Bangka Island Massacre, in which 22 Australian Army nurses, having survived the February 14, 1942, sinking of the SS Vyner Brooke after it had left Singapore, were herded into the surf and machine-gunned.

At the Japanese surrender in 1945, the US took the political decision to distance Emperor Hirohito from the vicious excesses of his subjects. Washington feared that holding the imperial family responsible for incidents such as the Rape of Nanking - where Prince Asaka, an uncle of the emperor, allegedly issued the order to kill all captives - would trigger unrest among the defeated population.

Wild, though, began to hear increasing allegations concerning the involvement of the emperor in the infamous Unit 731, which for 13 years carried out experiments on local civilians from its Harbin base, in northern China. The unit's tests included dropping ceramic bombs containing infected insects from aircraft and leaving out food contaminated with pathogens, knowing the local population would eat it. Pens and walking sticks were reportedly smeared with viruses that would be passed on to anyone who picked them up. They tied naked civilians, who they referred to as " murata" - "logs" - to stakes in the sub-zero Manchurian winter and sprayed them with water until they were frozen. Frostbite tests were then conducted to see how much force was required to shatter a limb.

Similar experiments were carried out across occupied Asia by related detachments, including the Singapore-based Unit 9420.

Sheldon Harris' Factories of Death: Japanese Biological Warfare, 1932-45, And the American Cover-up is considered the most authoritative book on imperial Japan's use of unconventional weapons in the early 20th century. He notes that Unit 731 was created by imperial edict in 1936 and that Hirohito - a trained scientist who published at least nine papers on hydroids in Japanese academic journals - had expressed an interest in the 1920s in chemical and biological warfare. There are suggestions that he personally funded Unit 731's research.

Hirohito also knew Major Shiro Ishii, the man described in Harris' book as, "The person most responsible for converting Manchuria into one huge biological warfare laboratory during the Japanese occupation."

Hirohito met the major at least twice in the 30s and participated in an experiment in which Ishii demonstrated a purifying device that turned urine into water (Unit 731 was a biological-warfare unit disguised as a water-purification operation). Ishii's unit was awarded meritorious citations by the emperor for its efforts in what Japan calls the Nomonhan Incident of 1939.

The US had heard about Unit 731 and investigators from Fort Detrick, the centre for the US military's biological weapons programme, were among the first to arrive in Tokyo after the 1945 surrender.

Over the next seven months, key members of the unit were taken in for questioning, including Ishii, who had attempted to avoid the dragnet by faking his own funeral.

The detentions and alleged crimes of dozens of scientists were reported widely in the media until in April 1946, when all mention of chemical, biological and bacteriological warfare ceased. The International Military Tribunal for the Far East began that month and, in the two years and seven months during which the Allies debated Japan's responsibility for the war, biological warfare was mentioned only once. It took up a mere 10 minutes of the court's time.

"The US got all the documents related to Unit 731 and germ warfare and, in return, Major Ishii and the others never faced trial," Takao Matsumura, a retired professor of history at Japan's Keio University, tells Post Magazine.

And Matsumura believes there are other secrets that have not come out.

"Most historians can't agree, but I think the emperor knew about Unit 731," he says. "His brother went to Unit 731's headquarters in Harbin and was photographed in front of the main gate."

There are many who believe that the Pentagon used what it learned from Unit 731 during the Korean war, which broke out five years later. To this day, North Korea claims the US subjected its troops and civilians to flies, beetles, spiders, crickets and fleas infected with a long list of pathogens - from plague bacillus to cholera, anthrax, encephalitis and yellow fever.

The US has repeatedly dismissed the allegations as "the disinformation campaign that refuses to die".

Back in 1945, however, not everyone was put off the investigation into Ishii and his cohorts so easily. Colonel Thomas Morrow, principal assistant to the chief US war crimes prosecutor, visited China and compiled a dossier on Unit 731's activities.

Shortly afterwards, he was reassigned to new duties in Washington and his investigation stalled. In 1951, at the height of the Korean war, Lieutenant Colonel Arvo Thompson, another investigator who had probed the Unit 731 allegations, died in what many believe were suspicious circumstances but was later ruled to be a suicide.

Wild was warned off his own inquiries.

"A few days before his death, Colonel Wild had made it known to the American authorities that he had enough evidence to be able to convict Hirohito of war crimes, including bacteriological warfare experiments," says Lane, who worked at the time on the switchboard in Britain that received all signals from the Far East. "He was then ordered to cancel any further work in this direction and to hand over all the documentation he had so far accumulated.

"It was made clear that he would have to take his orders from the commander of the occupation of Japan, which was General [Douglas] MacArthur," Lane claims. "Wild was ordered to cease all further investigations against any Japanese involved in bacteriological experiments."

That message had been conveyed very clearly to Wild during his final visit to Tokyo, in September 1946. He subsequently left Japan to attend a war crimes trial in Singapore, making a fatal stop-off en route in Hong Kong.

Lane believes Wild "was about to use the Singapore hearings to expose his information concerning the emperor". To have done so in a public forum and with the world's media watching would have shattered the protective cordon thrown up around the emperor and raised new, awkward questions about the knowledge the US had gained and what it had given up in return for it.

"I believe the aircraft was sabotaged on the orders of the American security services," he says.

Having received no replies or explanations from the Ministry of Defence in London to repeated requests for further details of the crash, Lane commissioned a self-funded report from a former RAF air-accident investigator, who pointed out that as the pilot and the co-pilot of the Dakota had nearly 2,000 hours flying time between them, they would have been aware of the possibility of crosswinds above the airfield.

"It is not possible to conceive how a crosswind would get the better of two experienced pilots and the only explanation might be that the Dakota suffered an engine failure," Joe Bamford, the investigator, stated in his final report, which was completed in October 2001.

"If the aircraft was in a climbing turn to port, for instance, and it suddenly lost the port engine, the power from the starboard engine would spin it into the ground," he wrote. "An explosion on one side or the other would have a similar effect.

"My suspicions are that a small explosive [device] had been attached to one of the engines."

That suggestion is supported by two Chinese ground engineers who testified before the crash inquiry in Hong Kong that they saw a "small flash" from the aircraft just before it fell from the sky.

Further arousing suspicion, all the documents from Wild's office in Tokyo were removed immediately after the accident and have not been seen since.

"From the beginning to the end, I believe that the British authorities have covered this incident up," Lane says.

Lane's requests for further information from the Ministry of Defence in London have been ignored, while attempts to speak to Wild's relatives have been rebuffed.

"In 1992, I attended the funeral of the padre who was a friend of Wild," he says. "I spoke to Wild's sister-in-law and she knew I was making inquiries.

"She insisted that I drop them," he says. "She said the Wild family had no desire to know the truth."