This summer marks the 60th anniversary of the start of the Korean War, a bloody three-year conflict that set Communist North Korea against a South Korea supported by a UN coalition headed by the US.
It was the first armed confrontation of the Cold War and by the time a truce was agreed in 1953, two million soldiers and two million civilians had been killed or wounded.
Six decades on, the conflict is still not formally resolved.
Troops from both sides continue to face each other across the 38th parallel, while the relationship between Washington and Pyongyang, the North Korean capital, is dominated by acrimonious quarrels over the latter's nuclear weapons programme.
But there is another bitter and intractable dispute that continues to haunt both sides.
North Korea alleges that the US used biological weapons against Korean civilians during the war– dropping "germ" bombs containing insects, shellfish and feathers infected with anthrax, typhoid and bubonic plague on villages across the country.
The US has always vehemently denied these claims, dismissing them as crude and outlandish communist propaganda from a secretive and totalitarian state.
Nevertheless, the accusations have refused to go away. Pyongyang continues to press for an apology for an "outrage" that the US insists never happened.
|Professor Mori Masataka has been trying to unravel the truth about alleged germ warfare
In a specially extended edition, People & Power set out to investigate this extraordinary story.
Our journey began in North Korea where we were given unprecedented access to follow a leading Japanese academic, Professor Mori Masataka, who has been trying to unravel the mystery for the last twenty years.
On this, his fourth visit to the country, Mori's intention was to talk to men who claim to have witnessed, first hand, biological attacks on villages in 1952.
But neither he nor People & Power's location producer, Tim Tate, were under any illusions.
North Korea is one of the world's most secretive states and is usually impenetrable to journalists. Everywhere our cameras went, government officials went too, strictly monitoring where and what we could film.
In a vast museum in the centre of Pyongyang, Mori explored a room given over to what the North Koreans claim is direct evidence of US germ warfare – including specimen jars filled with flies, mosquitoes and fleas all allegedly injected with deadly pathogens.
A smartly uniformed army officer, Captain Ryu Uk Hui, drew his attention to some salvaged bomb casings.
On impact, she said, they were adapted to split open and release the insects to infect the local population. A film-show followed.
|North Koreans said masses of insects crawled around bomb casings which fell in the snow
The grainy black and white footage, purportedly North Korean news film from 1952, appeared to show masses of insects crawling on the snow covered ground beside the bomb casings.
All this could have been phony, of course, and that is how the US has always responded to such claims, especially to filmed "confessions" from 36 captured US airmen - also screened in Pyongyang's museum - in which they give the North Koreans apparently detailed accounts of their participation in the US "germ" raids.
Accounts that, it must be said, were all retracted on the air crews' return home to the US after the war.
But other testimony is more difficult to fake convincingly.
Later, we are driven deep into the North Korean countryside, to a village called Hwanjin, where two elderly farmers are patiently waiting.
It is clear they have been tidied up for the occasion and both wore patriotic badges pinned to their tunics, yet their weathered faces, calloused hands and still grimy fingernails speak of long years spent in the fields.
Although it is impossible to be sure, neither seems to be a Communist Party apparatchik primed for the occasion. And one speaks with convincing passion about the events that took the life of his father and many others, in the days after the insects came.
"It was in March", says Yun Chang Bin. "The flies were big and their colour was brown-ish.
"Not long after that, about April, terrible epidemics like typhoid fever were spread. People in the village developed high temperatures. Loss of appetite and then aches on the arms and legs, there was much pain."
There were some 50 households in the village, he went on, and more than thirty people died.
"My father died. He suffered a high fever, and then he was not able to use the lower half of his body, he wasn't able to eat and was not able to move."
As his fellow farmer nods encouragingly beside him, Yun Chang Bin looks directly at Professor Mori.
"I want you to go and tell the peace-loving people in the world about the atrocity the Americans committed to inflict pain to us, to make us unhappy, to kill all us Korean people, by scattering germ bombs to exterminate us."
Tears and grimace
At another village, another eyewitness, Li San, Bum holds his arms out as he describe the iron bomb that almost six decades ago had tumbled out of a low flying plane onto a nearby frozen lake, spilling its cargo of insects out onto the snow. And then the villagers began to get sick and die.
"When they moved their bowels their stools had blood in them. And then they developed fever, and the fever made them vomit everything. My grandmother died after contracting this fever. One of my uncles died as well. So we should regard the Americans as arch enemies - how can we think well of them," Li San says.
Mori has interviewed dozens of North Koreans over the years and has heard similar tales from all of them. "They told me their stories, shedding tears and grimacing with anger. They told me this germ warfare actually happened."
|Yun Chang Bin says his father died from high fever after the US bombed near his village
But however convincing he has found these accounts, Mori knows that testimony from North Korean citizens will not be enough to convince a sceptical world that the US used germ warfare in Korea.
"A scientific investigation or medical or biological investigation should be carried out. I think it is definitely necessary that a non-political purely-scientific organisation should be sent to North Korea to investigate", Mori says.
As it happens, within months of the original allegations being made back in the 1950s, North Korea invited an international commission to visit the country.
Composed of scientists from France, Italy, Sweden, the Soviet Union and Brazil, and led by Joseph Needham, a distinguished - if left-leaning - British embryologist, the commission toured the affected areas, interviewed the sick and the dying and carried out a detailed analysis of their infections.
The resulting 600-page report included results of post-mortem on the victims: these identified bubonic plague, cholera and anthrax.
It concluded that germ warfare had been deployed exactly as the North Koreans claimed. Yet despite its apparent wealth of scientific evidence, it was again dismissed by the US as communist disinformation.
Which is why, if a new international enquiry was ever undertaken, it would have to spread its net far further than North Korea and to the US, in particular, where the truth almost certainly lies, buried deep in the Cold War secrets of a superpower.
It was there that People & Power discovered that during the 1940s and 1950s American scientists at the US Army base in Fort Detrick, Maryland, had developed ways of delivering bomb-loads of insects infected with bubonic plague and other deadly pathogens.
Our investigations also uncovered two remarkable documents in the US National Archives.
They revealed that the US had bought the expertise of Unit 731, a Japanese army biological warfare team, which conducted human experiments in the 1930s and 1940s to perfect the technology of bacteriological warfare: in World War 2, the Japanese military had dropped thousands of "germ bombs" across Northern China, killing millions of civilians.
A third crucial document - marked "Top Secret" - showed that in September 1951, the US Joint Chiefs of Staff issued orders to begin "large scale field tests ... to determine the effectiveness of specific BW [bacteriological warfare] agents under operational conditions."
If these "field tests" were indeed undertaken, then they may have drawn again on the expertise of the Japanese biological warfare team.
In Japan, People & Power found home video footage from one of the former members of that team, shot just before his death, in which he claimed that its leaders had indeed assisted the US in mounting "an attack" in Korea.
But perhaps the most telling evidence came from a former US air force officer who took part in bombing raids over North Korea.
Kenneth Enoch was shot down in January 1952 and held as a POW for 20 months.
While in captivity, he was one of 36 US air force officers who made written and filmed "confessions" that they had taken part in "germ bomb" missions.
When these POWs were repatriated in 1953, the US department of defence threatened to charge them with treason for co-operating with their captors.
Each then retracted their confessions in front of military cameras: each claimed they had been tortured or indoctrinated by North Korean and Chinese guards.
But when we tracked down and interviewed Enoch, now a sprightly 85 and living in a gated retirement community in Texas, he denied having been ill-treated or indoctrinated – and appeared to make at least a partial admission that the US did use biological weapons in the Korean War.
"The people who deal in that don't have to go and fight, and that's a pretty sweet deal for them. You know, but they send it with you," he said. Nevertheless, he continued to deny that he personally played any part in biological weapons attacks.
|At one point, Enoch said his statements had been coerced by the North Koreans
Records of Enoch's bombing missions over North Korea were removed by US air force investigators from the official records in March 1952 – two months after he was captured and one week before he made his confession to "germ warfare".
People & Power asked both the US state department and the department of defence for an interview about the issue raised in our film.
They turned down the offer and also declined to answer ten specific questions we put to them about North Korea's allegations.
Instead, a spokesman for the US administration dismissed the claims as "baseless" and said they were "the disinformation campaign that refuses to die."
So who is to be believed? Professor Mori Masataka, thinks he knows the answer. "Use of germ weapons in war is in breach of the Geneva Convention. I think that's why the Americans are refusing to admit the allegations. But I have no doubt. I'm absolutely sure that this happened."
The clear implication, of course, is that were North Korea's claims ever to be proved, the US might be open to prosecution for war crimes – which would be awkward, to say the least, at a time when the US is relying on its moral authority to underpin international efforts to combat global terrorism and nuclear proliferation.
Either way, one thing is clear. Until the allegations are laid to rest and the US's innocence or culpability is established beyond doubt - perhaps by an independent enquiry – one of the most enduring Cold War mysteries will continue to haunt Washington's relationship with the world's most secretive state.
This episode of People & Power aired from Wednesday, March 10, 2010.