Covid-19 most likely leaked from a virus lab in Wuhan, The US Department of Energy has concluded according to a new document.
A classified intelligence report provided to White House officials last week suggested the agency now believes an accidental exposure within the Chinese city is the most likely cause.
The new finding - classified as being made with "low confidence" - sees the Energy Department join the FBI in lending support to the once-dismissed "lab leak" theory as the source of the global pandemic.
The Wuhan Institute of Virology (WIV) research facility is less than 10 miles from an animal slaughter market where the first series of human cases were clustered.
There is not yet any evidence for a "lab leak".
They had previously remained undecided on the origin of the virus.
But their change in position is noted in a new update to a 2021 document on coronavirus produced by the office of the Director of National Intelligence, according to The Wall Street Journal.
Opinion on how Covid-19 began remains divided across different arms of the US government, with four other agencies and a national intelligence panel believing it resulted from natural transmission.
Two other agencies are still undecided.
US national security adviser Jake Sullivan refused to confirm or deny the reports during an appearance on the Sunday on CNN show, and instead chose to emphasise President Joe Biden's push to discover the origin of the virus.
He said: “President Biden specifically requested that the national labs, which are part of the Energy Department, be brought into this assessment because he wants to put every tool at use to be able to figure out what happened here,”
"Some elements of the intelligence community have reached conclusions on one side, some on the other. A number of them have said they just don’t have enough information to be sure"
Mr Sullivan added that "if we gain any further insight or information" on the subject the government would share it with the American people, but stressed that there was not currently a "definitive answer that has emerged from the intelligence community on this question".
British, American and other Western intelligence agencies had almost universally talked down the likelihood of a lab leak at the beginning of the Covid pandemic before a larger contingent reportedly began to believe it was "feasible" around the spring of 2021.
Covid is believed to have jumped from animals to humans at Huanan Seafood Market, where animals known to harbour Covid including raccoon dogs, hedgehogs, rats and squirrels, were kept in cramped conditions.
