US intelligence agencies have found no direct evidence that Covid-19 broke out from a Chinese laboratory, a declassified report has said.
The Office of the Director of National Intelligence (ODNI) said both a natural and laboratory origin remain plausible scenarios.
It noted most US intelligence agencies agree the virus was not genetically engineered or “laboratory-adapted”.
The origins of the pandemic have been a matter of bitter debate in the US.
The lab-leak theory has been strongly rejected by China.
The ODNI report was released on Friday night after Congress passed a bill in March giving US intelligence 90 days to declassify what it knew about the Wuhan Institute of Virology (WIV).
“The Central Intelligence Agency and another agency remain unable to determine the precise origin of the Covid-19 pandemic, as both (natural and lab) hypotheses rely on significant assumptions or face challenges with conflicting reporting,” said the 10-page report.
Four agencies believe the virus was transferred from animals to humans, said the findings.
Two other agencies – the Energy Department and the FBI – continue to believe that the virus leaked from a lab.
The findings said the US intelligence community still could not rule out the possibility that the virus came from a laboratory.
All US intelligence agencies agree that Covid-19 was not developed as a biological weapon, said the report.
It said that the WIV and Chinese army had collaborated in work on coronaviruses for public health needs, although not on any infections that could plausibly have spawned Covid-19.
But the report also noted that scientists at the WIV have engineered “chimeras”, or combinations of coronaviruses, and used reverse genetic cloning techniques that could hide intentional changes.
The findings said that some researchers probably did not use adequate biosafety precautions before the pandemic.
However, US intelligence is not aware of a specific biosafety incident at the Wuhan lab that caused a Covid-19 outbreak, according to the report.
Several researchers there were ill in autumn of 2019, but their sickness could have been caused by a number of diseases and some of their symptoms were not consistent with Covid-19, it said.