The FBI's director has added his support to claims COVID-19 leaked from a lab in China.
The theory was dismissed by some as disinformation during the initial months of the pandemic.
But it has gained credibility in some quarters, despite angry denials from China.
Once dismissed as a wild conspiracy theory, the claim that COVID-19 leaked from a laboratory in Wuhan, China, is gaining traction.
FBI Director Christopher Wray on Tuesday lent his support to the claim in an interview with Fox News, saying the virus "most likely" originated in a "Chinese government-controlled lab."
"The FBI has for quite some time now assessed that the origins of the pandemic are most likely a potential lab incident," he said, in the first public confirmation of the FBI's classified judgement on the source of the pandemic.
It came days after the US Energy Department found a lab leak to be the most likely source of the disease— although it said it reached that conclusion with a low level of confidence. In response, Beijing accused Washington of "political manipulation".
China has pushed another theory, suggesting the COVID-19 may have jumped to humans from frozen food shipped from elsewhere in the world.
So how has the claim that coronavirus originated in a Chinese lab found its way into the mainstream?
Lab leak theory initially dismissed
The suspicion that COVID-19 may have leaked from a Wuhan lab has circulated since the earliest days of the pandemic.
The theories focus on the fact that the Wuhan Institute of Virology, where research into coronaviruses has long been conducted, is located a short distance from the market from which the first cases were linked.
When former President Donald Trump began promoting the theory in April, 2020, it became the subject of fierce partisan debate in the US, with some dismissing it as a conspiracy. Trump sought to use the pandemic to discredit China, using the xenophobic term "China virus" to describe the disease.
Anthony Fauci, the US government's top expert on infectious diseases, said in 2020 that he believed the virus had originated in nature, but more recently he's expressed doubt we'll ever know the source of the virus.
China's lack of transparency fuels rumors
China has since the earliest days of the pandemic resisted a full and transparent investigation into its origins, fuelling rumors and speculation.
Beijing has strongly pushed back against the claim the virus could've originated in a lab, describing the hypothesis as having no scientific basis.
China has pushed its own conspiracy theories about the source of the virus, claiming it could have originated in US labs, a claim for which it's offered no evidence.
When Australia pushed for international inspectors to allow access to China in 2020 to investigate COVID-19's origins, Beijing responded with sweeping economic sanctions.
However, China did allow World Health Organization (WHO) investigators into the country as part of a joint probe. The March 2021 WHO report found that it most likely originated in nature, and described the lab leak theory as "extremely unlikely," but added that China had not been fully cooperative. A group of scientists criticised the WHO for dismissing the lab leak thesis too hastily, and pointed to gaps in the report's evidence.
China has rejected calls for a follow-up investigation, and has pointed to a report by one of its leading virologists who found several coronavirus strains which could make the leap to humans in bat caves near Wuhan in 2015.
Yet the lab leak theory has continued to gain credibility, despite China's efforts, and scientists who once dismissed it now think it's a credible explanation.
We may never know the truth
In May 2021, US media reported that intelligence agencies had found that several employees of the Wuhan lab had been admitted to hospital in November 2019, weeks before the first recorded cases of COVID-19. Shortly after, President Joe Biden ordered intelligence agencies to renew their efforts to discover the origins of the virus.
But for now, the prospect of researchers being allowed to conduct a new on-the-ground study in China appear remote, and a conclusive answer to the mystery seems no closer.
The Chinese foreign ministry on Tuesday rejected the US ambassadors call for more transparency from China on COVID-19's origins. Mao Ning called on US investigators to "stop smearing China and stop politicizing origins-tracing."
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