The possibility the Covid virus leaked from a laboratory should not be ruled out, a former top Chinese government scientist has told BBC News.
As head of China's Centre for Disease Control (CDC), Prof George Gao played a key role in the pandemic response and efforts to trace its origins.
China's government dismisses any suggestion the disease may have originated in a Wuhan laboratory.
But Prof Gao is less forthright.
In an interview for the BBC Radio 4 podcast Fever: The Hunt for Covid's Origin, Prof Gao says: "You can always suspect anything. That's science. Don't rule out anything."
A world-leading virologist and immunologist, Prof Gao is now vice-president of the National Natural Science Foundation of China after retiring from the CDC last year.
In a possible sign that the Chinese government may have taken the lab leak theory more seriously than its official statements suggest, Prof Gao also tells the BBC some kind of formal investigation into the Wuhan Institute of Virology (WIV) was carried out.
"The government organised something," he says, but adds that it did not involve his own department, the China CDC.
We asked him to clarify whether that meant another branch of government carried out a formal search of the WIV - one of China's top national laboratories, known to have spent years studying coronaviruses.
"Yeah," he replies, "that lab was double-checked by the experts in the field."
It's the first such acknowledgement that some kind of official investigation took place, but while Prof Gao says he has not seen the result, he has "heard" that the lab was given a clean bill of health.
"I think their conclusion is that they are following all the protocols. They haven't found [any] wrongdoing."
The virus that causes Covid, it is almost certain, once came from bats.
But how it got from bats to us is a far more controversial question, and from the start there were two main possibilities.
One is that the virus spread naturally from bats to humans, perhaps via other animals. Many scientists say the weight of evidence suggests that is the most likely scenario.
But other scientists say there is not enough evidence to rule out the main alternative possibility - that the virus infected someone involved in research which was designed to better understand the threat of viruses emerging from nature.
Those two alternatives now find themselves at the heart of a geopolitical stand-off, a swirling mass of conspiracy theories, and one of the most politicised and toxic scientific debates of our time.
In the new BBC podcast we shed light on this difficult, but vitally important, question through interviews with some of the leading scientists from all sides of the debate - as well as on-the-ground reporting, from the streets of Wuhan to the inside of a high-security laboratory in the US.
A Singapore-based scientist, Prof Wang Linfa, was visiting the Wuhan Institute of Virology (WIV), where he is an honorary professor, in January 2020, just as the coronavirus outbreak was taking hold.
He tells the BBC a colleague at the WIV had been worried about the possibility of a lab leak, but that she was able to dismiss it.
Prof Wang is a professor of emerging infectious diseases at the Duke-NUS Medical School in Singapore, and collaborates regularly with Prof Shi Zhengli, a professor with the same speciality at the WIV.
Long-standing friends, they are two of the world's top experts on bat coronaviruses - earning themselves the nicknames Batman and Batwoman.
Prof Wang says Prof Shi told him she "lost sleep for a day or two" because she worried about the possibility that "there's a sample in her lab that she did not know of, but has a virus, contaminated something, and got out".
But he says that she checked her samples and found they contained no evidence of the virus that causes Covid or any other virus close enough to have caused the outbreak.
He also says there's "zero chance" that Prof Shi or anyone in her team was hiding the fact that they had found evidence of a lab leak because they were behaving like nothing happened, including going out for dinner, and planning a karaoke session.
Now-declassified US intelligence suggests that several researchers at the WIV became sick in autumn 2019 with symptoms "consistent with both Covid-19 and common seasonal illnesses".
But Prof Wang tells us that he suggested Prof Shi take blood samples from her team to see if they had Covid antibodies in January 2020. He says she followed his advice and all the tests were negative.
Prof Wang is one of a group of scientists who believe that the evidence overwhelmingly suggests that the virus passed to humans in a Wuhan market.
The Huanan Seafood Market - which sold much more than its name suggests, including wild mammals - was connected to many of the early cases, people who worked or shopped there.
Although China has shown a marked lack of transparency, those scientists say there is now enough information, such as the data on those early cases and the environmental sampling in that market, to rule out a lab leak.
In fact, such claims of certainty have been there from the start, most notably in a March 2020 paper which has become one of the most read and most controversial scientific papers of the internet age.
"The Proximal Origin of Sars-Cov-2" was written by some of the most eminent scientists in the field of virology and emerging disease, and it concluded: "We do not believe that any type of laboratory-based scenario is plausible."
It helped to bolster the idea - that quickly became prevalent in much of the media coverage - that the lab leak was a conspiracy theory.
But one of the paper's authors has told the podcast that he now has doubts about the strength of that earlier conclusion.
Ian Lipkin, a professor of epidemiology at Columbia University in New York, has long-experience tracking diseases around the world, including in China, where he has built strong contacts.
He was also the scientific adviser on the Hollywood blockbuster Contagion.
Prof Lipkin now says ruling out any lab-based scenario in the paper was putting it too strongly.
While he continues to believe that the market remains the most plausible explanation for where Covid came from, and does not believe the virus was deliberately engineered, he does not feel all laboratory or research scenarios can yet be excluded.
And he volunteers a theory of his own, pointing to another Wuhan laboratory - run by the Wuhan Centre for Disease Control - located just a few hundred metres away from the Huanan Seafood Market.
It was known to be involved in the collection of thousands of blood and faecal samples from wild bats, research that was sometimes done without wearing proper protective equipment, according to Chinese news reports - a clear infection risk.
"The people who work there could have become infected while they're in a cave collecting bats," Prof Lipkin says, adding that he was not aware of the lab and its work when he co-wrote the March 2020 paper.
Prof Lipkin says that further analysis pointing to the Huanan Seafood Market as the origin of the virus - including recent research focused on evidence of raccoon dogs at the market - does not resolve the origin question.
The virus, he says, could have "originated outside of the market and been amplified in the market".
On the surface, Prof Gao's comments about not ruling out a lab leak appear seriously at odds with China's publicly stated position.
Risky even.
"The so-called 'lab leak' is a lie created by anti-China forces. It is politically motivated and has no scientific basis," reads a statement provided by the Chinese embassy in the UK.
But looked at another way, there may be more common ground than it seems.
In its propaganda, the Chinese government has been pushing a strange, unsubstantiated third theory of its own.
The virus, it says, didn't come from the lab or the market but may have been brought into the country on frozen food packaging.
The Chinese government says it rules out both the lab and the market - and Prof Gao's comments could simply be seen as the more scientific version of that position, because he rules out neither. Both are based on that idea of a lack of evidence.
"We really don't know where the virus came from… the question is still open," Prof Gao tells the BBC.
Scientists dispute - sometimes bitterly - whether the question really is still open.
But, outside China at least, there is broad agreement on one thing: China has not done enough to look for evidence or share it.
Though it may seem like a simple question, it's anything but.
Where did Covid come from?
For every life lost, for everyone who's suffered and for those who continue to suffer, the answer matters.
The Fever: The Hunt for Covid's Origin podcast is available on BBC Sounds.