DR Anthony Fauci has admitted he is "not convinced" Covid-19 developed naturally.
The called for an investigation into "what went on in China" as "we need to find out the truth".
At the United Facts of America: A Festival of Fact-Checking event, PolitiFact’s Katie Sanders asked Dr Fauci: "There’s a lot of cloudiness around the origins of Covid-19 still, so I wanted to ask, are you still confident that it developed naturally?"
The top doc responded: "No actually. I am not convinced about that.
"I think we should continue to investigate what went on in China until we continue to find out to the best of our ability what happened."
He continued: "Certainly, the people who investigated it say it likely was the emergence from an animal reservoir that then infected individuals, but it could have been something else, and we need to find that out.
"So, you know, that’s the reason why I said I’m perfectly in favor of any investigation that looks into the origin of the virus."
Sen Rand Paul asked Dr Fauci during a Senate committee hearing last Tuesday: "Will you in front of this group categorically say that the Covid-19 virus could not have occurred by serial passage in a laboratory?"
Dr Fauci replied: "I do not have any accounting of what the Chinese may have done, and I'm fully in favor of any further investigation of what went on in China.
"However, I will repeat again, the National Institutes of Health and National Institute of Allergy and Infectious Diseases categorically has not funded gain of function research to be conducted in the Wuhan Institute of Virology."
Sen Paul and Dr Fauci clashed during the meeting, which discussed grants by the NIH to the Wuhan lab - from which many believe coronavirus emerged.
FAUCI FLIPS HIS STANCE
Dr Fauci's most recent remarks suggest he has drastically altered his stance on how Covid spread since the pandemic began.
In May 2020, he dismissed the lab leak theory, telling : "If you look at the evolution of the virus in bats and what’s out there now, [the scientific evidence] is very, very strongly leaning toward this could not have been artificially or deliberately manipulated.
"Everything about the stepwise evolution over time strongly indicates that [this virus] evolved in nature and then jumped species."
After a team from the World Health Organization (WHO) visited China - including the Wuhan lab - it was concluded that it was "extremely unlikely" that Covid originated from there.
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But Republican lawmakers on the House Intelligence Committee said on Wednesday that there is “overwhelming circumstantial evidence” that Covid-19 came from China’s Wuhan Institute of Virology.
The White House has joined calls for an independent investigation in Covid's origins.
Press secretary Jen Psaki said this week the Biden administration has conveyed both publicly and privately that the Chinese government was "not transparent from the beginning".