FBI Director Christopher Wray has said that the bureau believes Covid-19 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 told Fox News.
It is the first public confirmation of the FBI's classified judgement of how the pandemic virus emerged.
Many scientists point out there is no evidence that it leaked from a lab.
nd other US government agencies have drawn differing conclusions to the FBI's.
Some of them have said - but with a low level of certainty - that the virus did not start in a lab but instead jumped from animals to humans.
The White House has said there is no consensus across the US government on the origins.
A joint China-World Health Organization (WHO) investigation in 2021 called the lab leak theory "extremely unlikely".
However, the WHO investigation was deeply criticised and its director-general has since called for a new inquiry, saying: "All hypotheses remain open and require further study."
Mr Wray's comments come a day after the US ambassador to China called for the country to "be more honest" about Covid's origins.
In his interview on Tuesday, Mr Wray said China "has been doing its best to try to thwart and obfuscate" efforts to identify the source of the global pandemic.
He said details of the agency's investigation were classified but the FBI had a team of experts focusing on the dangers of biological threats.
In response, Beijing accused Washington of "political manipulation".
"The conclusions they have reached have no credibility to speak of," said Chinese foreign ministry spokeswoman Mao Ning.
Some studies suggest the virus made the leap from animals to humans in Wuhan, China, possibly at the city's seafood and wildlife market.
The market is near a world-leading virus laboratory, the Wuhan Institute of Virology, which conducted research into coronaviruses.
A few days ago, the US Department of Energy said it had found the virus was most likely the result of a lab leak in Wuhan but could only reach that conclusion with "low confidence".
In response to that, many scientists who have studied the virus said this week that there is no new scientific evidence pointing to a lab leak.
A natural origin is still the more likely theory, said Professor David Robertson, head of viral genomics and bioinformatics at University of Glasgow.
"There's been an accumulation of evidence (what we know about the viruses biology, the close variants circulating in bats and locations of early human cases) that firmly points to a natural origin centred on the Huanan market in Wuhan city," he said.
On Monday, White House National Security Council spokesman John Kirby said that US President Joe Biden supports "a whole-of-government effort" to discover how Covid began.
"We're just not there [at consensus] yet," he said. "If we have something that is ready to be briefed to the American people and the Congress, we will do that."
Tensions in bilateral ties between the US and China have spiked in the wake of the recent spy balloon saga.
A bipartisan panel of US lawmakers this week kicked off a series of hearings on the "existential" threat of the ruling Chinese Communist Party.
The first session of the House Select Committee on Strategic Competition between the United States and the Chinese Communist Party focused on issues such as human rights and the US economy's dependence on Chinese manufacturing.
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