The coronavirus pandemic probably arose from a Chinese laboratory leak, the US Department of Energy announced on Sunday, although sources who have seen its classified report said the agency made its judgment with "low confidence".
The White House maintained that US intelligence remained divided on the issue.
The Wall Street Journal reported that the conclusion marks a shift by the energy department, which had previously said it was undecided how the virus emerged.
The conclusion, reportedly the result of new intelligence, is nevertheless significant because the department oversees a network of national laboratories, including some that conduct advanced biological research.
The FBI believes that the pandemic, which has left nearly seven million people dead, was the result of a mishap in a Chinese laboratory.
But four US intelligence agencies believe Covid occurred through natural transmission, while two others remain undecided, the Journal reported.
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White House National Security Adviser Jake Sullivan stressed that a "variety of views" on the matter remain.
"Right now, there is not a definitive answer that has emerged from the intelligence community on this question," Mr Sullivan told CNN.
In mid-February, the World Health Organisation pledged to do everything possible "until we get the answer" on Covid's origins, denying a report saying it had abandoned its investigation.
The scientific community considers it crucial to determine the origins of the pandemic to better fight or even prevent the next one.