A Chinese scientist at the centre of unsubstantiated claims that the coronavirus leaked from her laboratory in the Chinese city of Wuhan has told the BBC she is open to "any kind of visit" to rule it out. The surprise statement from Prof Shi Zhengli comes as a World Health Organization team prepares to travel to Wuhan next month to begin its investigation into the origins of Covid-19.

The remote district of Tongguan, in China's south-western province of Yunnan, is hard to reach at the best of times. But when a BBC team tried to visit recently, it was impossible.

Plain-clothes police officers and other officials in unmarked cars followed us for miles along the narrow, bumpy roads, stopping when we did, backtracking with us when we were forced to turn around.

We found obstacles in our way, including a "broken-down" lorry, which locals confirmed had been placed across the road a few minutes before we arrived.

And we ran into checkpoints at which unidentified men told us their job was to keep us out.

At first sight, all of this might seem like a disproportionate effort given our intended destination, a nondescript, abandoned copper mine in which, back in 2012, six workers succumbed to a mystery illness that eventually claimed the lives of three of them.

But their tragedy, which would otherwise almost certainly have been largely forgotten, has been given new meaning by the Covid-19 pandemic.

Those three deaths are now at the centre of a major scientific controversy about the origins of the virus and the question of whether it came from nature, or from a laboratory.

And the attempts of Chinese authorities to stop us reaching the site are a sign of how hard they're working to control the narrative.

For more than a decade, the rolling, jungle-covered hills in Yunnan - and the cave systems within - have been the focus of a giant scientific field study.

It has been led by Prof Shi Zhengli from the Wuhan Institute of Virology (WIV).

Prof Shi won international acclaim for her discovery that the illness known as Sars, which killed more than 700 people in 2003, was caused by a virus that probably came from a species of bat in a Yunnan cave.

Ever since, Prof Shi - often referred to as "China's Batwoman" - has been in the vanguard of a project to try to predict and prevent further such outbreaks.

By trapping bats, taking faecal samples from them, and then carrying those samples back to the lab in Wuhan, 1,600km (1,000 miles) away, the team behind the project has identified hundreds of new bat coronaviruses.

But the fact that Wuhan is now home to the world's leading coronavirus research facility, as well as the first city to be ravaged by a pandemic outbreak of a deadly new one, has fuelled suspicion that the two things are connected.

The Chinese government, the WIV, and Prof Shi have all angrily dismissed the allegation of a virus leak from the Wuhan lab.

But with scientists appointed by the World Health Organization (WHO) scheduled to visit Wuhan in January for an inquiry into the origin of the pandemic, Prof Shi - who has given few interviews since the pandemic began - answered a number of BBC questions by email.

"I have communicated with the WHO experts twice," she wrote, when asked if an investigation might help rule out a lab leak and end the speculation. "I have personally and clearly expressed that I would welcome them to visit the WIV," she said.

To a follow-up question about whether that would include a formal investigation with access to the WIV's experimental data and laboratory records, Prof Shi said: "I would personally welcome any form of visit based on an open, transparent, trusting, reliable and reasonable dialogue. But the specific plan is not decided by me."

The BBC subsequently received a call from the WIV's press office, saying that Prof Shi was speaking in a personal capacity and her answers had not been approved by the WIV.

The BBC denied a request to send the press office a copy of this article in advance.

Many scientists believe that by far the most likely scenario is that Sars-Cov-2, the virus that causes Covid-19, jumped naturally from bats to humans, possibly via an intermediary species. And despite Prof Shi's offer, for now there appears to be little chance of the WHO inquiry looking into the lab-leak theory.

The terms of reference for the WHO inquiry make no mention of the theory, and some members of the 10-person team have all but ruled it out.

Peter Daszak, a British zoologist, has been chosen as part of the team because of his leading role in a multimillion dollar, international project to sample wild viruses.

It has involved close collaboration with Prof Shi Zhengli in her mass sampling of bats in China, and Dr Daszak previously called the lab-leak theory a "conspiracy theory" and "pure baloney".

"I've yet to see any evidence at all of a lab leak or a lab involvement in this outbreak," he said. "I have seen substantial evidence that these are naturally occurring phenomena driven by human encroachment into wildlife habitat, which is clearly on display across south-east Asia."

Asked about seeking access to the Wuhan lab to rule the lab-leak theory out, he said: "That's not my job to do that.

"The WHO negotiated the terms of reference, and they say we're going to follow the evidence, and that's what we've got to do," he added.

One focus of the inquiry will be a market in Wuhan which was known to be trading in wildlife and was linked to a number of early cases, though the Chinese authorities appear to have already discounted it as a source of the virus.

Dr Daszak said the WHO team would "look at those clusters of cases, look at the contacts, look at where the animals in the market have come from and see where that takes us".

The deaths of the three Tongguan workers following exposure to a mineshaft full of bats raised suspicions that they'd succumbed to a bat coronavirus.

It was exactly the kind of animal-to-human "spillover" that was driving the WIV to sample and test bats in Yunnan.

It is no surprise then that, following those deaths, the WIV scientists began sampling bats in the Tongguan mineshaft in earnest, making multiple visits over the next three years and detecting 293 coronaviruses.

But apart from one brief paper, very little was published about the viruses they collected on those trips.

In January this year, Prof Shi Zhengli became one of the first people in the world to sequence Sars-Cov-2, which was already spreading rapidly through the streets and homes of her city.

She then compared the long string of letters representing the virus's unique genetic code with the extensive library of other viruses collected and stored over the years.

And she discovered that her database contained the closest known relative of Sars-Cov-2.

RaTG13 is a virus whose name has been derived from the bat it was extracted from (Rhinolophus affinis, Ra), the place it was found (Tongguan, TG), and the year it was identified, 2013.

Seven years after it was found in that mineshaft, RaTG13 was about to become one of the most hotly contested scientific subjects of our time.

There have been many well-documented cases of viruses leaking from labs. The first Sars virus, for example, leaked twice from the National Institute of Virology in Beijing in 2004, long after the outbreak had been brought under control.

The practice of genetically manipulating viruses is also not new, allowing scientists to make them more infectious or more deadly, so they can assess the threat and, perhaps, develop treatments or vaccines.

And from the moment it was isolated and sequenced, scientists have been struck by the remarkable ability of Sars-Cov-2 to infect humans.

The possibility that it acquired that ability as a result of manipulation in a laboratory was taken seriously enough for an influential group of international scientists to address it head on.

In what has become the definitive paper ruling out the possibility of a lab leak, RaTG13 has a starring role.

Published in March in the magazine Nature Medicine, it suggests that if there had been a leak, Prof Shi Zhengli would have found a much closer match in her database than RaTG13.

While RaTG13 is the closest known relative - at 96.2% similarity - it is still too distant to have been manipulated and changed into Sars-Cov-2.

Sars-Cov-2, the authors concluded, was likely to have gained its unique efficiency through a long, undetected period of circulation in humans or animals of a natural and milder precursor virus that eventually evolved into the potent, deadly form first detected in Wuhan in 2019.

Where though, some scientists are beginning to wonder, are those reservoirs of earlier natural infection?

Dr Daniel Lucey is a physician and infectious disease professor at the Georgetown Medical Centre in Washington DC and a veteran of many pandemics - Sars in China, Ebola in Africa, Zika in Brazil.

He is certain that China has already conducted thorough searches for evidence of precursor viruses in stored human samples in hospitals and in animal populations.

"They have the capability, they have the resources and they have the motivation, so of course they've done the studies in animals and in humans," he said.

Finding the origin of an outbreak was vital, he said, not just for wider scientific understanding, but also to stop it emerging again.

"We should search until we find it. I think it's findable and I think it's quite possible it's already been found," he said. "But then the question arises, why hasn't it been disclosed?"

Dr Lucey still believes that Sars-Cov-2 is most likely to have a natural origin, but he does not want the alternatives to be so readily ruled out.

"So here we are, 12, 13 months out since the first recognised case of Covid-19 and we haven't found the animal source," he said. "So, to me, it's all the more reason to investigate alternative explanations."

Might a Chinese laboratory have had a virus they were working on that was genetically closer to Sars-Cov-2, and would they tell us now if they did? "Not everything that's done is published," Dr Lucey said.

It's a point I put to Peter Daszak, the member of the WHO origins study team.

"You know, I've worked with the WIV for a good decade or more," he said. "I know some of the people there pretty well and I have visited the labs frequently, I've met and had dinner with them over 15 years.

"I'm working in China with eyes wide open, and I'm racking my brain back in time for the slightest hint of something untoward. And I've never seen that."

Asked if those friendships and funding relationships with the WIV presented a conflict of interest with his role on the inquiry, he said: "We file our papers; it's all there for everyone to see."

And his collaboration with the WIV, he said, "makes me one of the people on the planet who knows the most about the origins of these bat coronaviruses in China".

China may have provided only limited data about its hunt for the origin of Sars-Cov-2, but it has begun to promote a theory of its own.

Based on a few inconclusive studies conducted by scientists in Europe that suggest Covid-19 may have been circulating earlier than previously thought, state propaganda is full of stories suggesting the virus didn't start in China at all.

In the absence of proper data, speculation is only likely to grow, much of it focused on RaTG13 and its origins in a Tongguan mineshaft. Old academic papers have been dug up online that appear to differ from the WIV's statements about the sick mine workers - among them a thesis by a student at the Kunming Hospital University.

"I've just downloaded the Kunming Hospital University student's masters thesis and read it," Prof Shi told the BBC.

"The narrative doesn't make sense," she said. "The conclusion is neither based on evidence nor logic. But it's used by conspiracy theorists to doubt me. If you were me, what you would do?"

Prof Shi has also faced questions about why the WIV's online public database of viruses was suddenly taken offline.

She told the BBC that the WIV's website and the staff's work emails and personal emails had been attacked, and the database taken offline for security reasons.

"All our research results are published in English journals in the form of papers," she said. "Virus sequences are saved in the [US-run] GenBank database too. It's completely transparent. We have nothing to hide."

There are important questions to be asked in the Yunnan countryside, not just by scientists, but by journalists too.

After a decade of sampling and experimenting on viruses collected from bats, we now know that back in 2013 the closest known ancestor was discovered of a future threat that would claim well over a million lives and devastate the global economy.

Yet the WIV, according to the published information, did nothing with it, except sequence it and enter it into a database.

Ought that to call into question the very premise on which the expensive, and some would say risky, mass sampling of wild viruses is based?

"To say that we didn't do enough is absolutely correct," Peter Daszak told the BBC. "To say that we failed is not fair at all. What we should have been doing is 10 times the amount of work on these viruses."

Both Dr Daszak and Prof Shi are adamant that pandemic prevention research is vital, urgent work.

"Our research is forward-looking, and it's difficult for non-professionals to understand," Prof Shi wrote by email. "In the face of countless micro-organisms that exist in nature, we humans are very small."

The WHO is promising an "open-minded" inquiry into the origins of the novel coronavirus, but the Chinese government is not keen on questions, at least not from journalists.

After leaving Tongguan, the BBC team tried to drive a few hours north to the cave where Prof Shi carried out her ground-breaking research on Sars almost a decade ago.

Still being followed by several unmarked cars, we hit another roadblock, and were told there was no way through.

A few hours later, we discovered that local traffic had been diverted onto a dirt track that skirted the obstruction, but as we attempted to use the same route, we met yet another "broken down" car in our path.

We were trapped in a field for over an hour, before finally being forced to head for the airport.