The C.I.A. has said for years that it did not have enough information to conclude whether the Covid pandemic emerged naturally from a wet market in Wuhan, China, or from an accidental leak at a research lab there.

But the agency issued a new assessment this week, with analysts saying they now favor the lab theory.

That shift is based on “the available body of reporting,” although the other theory remains plausible, a spokeswoman for the agency said, adding that the agency will continue to evaluate any available credible new intelligence reporting.

Some American officials say the debate matters little: The Chinese government failed to either regulate its markets or oversee its labs. But others argue it is an important intelligence and scientific question.

John Ratcliffe, the new director of the C.I.A., has long favored the lab leak hypothesis. He has said it is a critical piece of intelligence that needs to be understood and that it has consequences for U.S.-Chinese relations.

The announcement of the shift came shortly after Mr. Ratcliffe told Breitbart News he no longer wanted the agency “on the sidelines” of the debate over the origins of the Covid pandemic. Mr. Ratcliffe has long said he believes that the virus most likely emerged from the Wuhan Institute of Virology.

Officials said the agency was not bending its views to a new boss, and that the new assessment had been in the works for some time.

In the final weeks of the Biden administration, Jake Sullivan, the national security adviser, ordered a new classified review of the pandemic’s origin. As part of that review, the agency’s previous director, William J. Burns, told analysts that they needed to take a position on the origins of Covid, though he was agnostic on which theory they should embrace, a senior U.S. intelligence official said.

Another senior U.S. official said it was Mr. Ratcliffe’s decision to declassify and release the new analysis.

There is no new intelligence behind the agency’s shift. Rather it is based on the same evidence it has been chewing over for months.

The analysis, however, is based in part on a closer look at the conditions in the high security labs in Wuhan province before the pandemic outbreak, according to people familiar with the agency’s work.

Since the outbreak of the pandemic, questions have swirled around whether the two labs handling coronaviruses in Wuhan had followed safety protocols strictly enough.

The agency made its new assessment with “low confidence,” which means the intelligence behind it is fragmentary and contradictory.

Even in the absence of hard intelligence, the lab leak hypothesis has been gaining ground inside spy agencies. But some analysts question the wisdom of shifting a position in absence of new information.

Former officials say they are not averse to a new examination of the Covid origins intelligence by the Trump administration. President Biden ordered a new review of the intelligence early in his administration after officials told the White House they had still-unexamined evidence.

Mr. Ratcliffe has raised questions about politicization in the intelligence agencies. Mr. Ratcliffe, who was the director of national intelligence in the first Trump administration, argued in an essay for Fox News in 2023 that the C.I.A. did not want to embrace the lab leak to avoid geopolitical problems for the Biden administration.

“The real problem is, the only assessment the agency could make — which is that a virus that killed over a million Americans originated in a C.C.P.-controlled lab whose research included work for the Chinese military — has enormous geopolitical implications that the Biden administration does not want to face head-on,” he said in the piece, which was written with Cliff Sims, a top aide. C.C.P. refers to China’s Communist Party.

Mr. Ratcliffe said on Thursday, when he was sworn in, that a look at the origins of Covid was a “Day 1” priority.

“I think our intelligence, our science and our common sense all really dictate that the origins of Covid was a leak at the Wuhan Institute of Virology,” he told Breitbart. “But the C.I.A. has not made that assessment or at least not made that assessment publicly. So I’m going to focus on that and look at the intelligence and make sure that the public is aware that the agency is going to get off the sidelines.”

Senior intelligence officials in the Biden administration defend their process and methodology. They have said that no intelligence was suppressed and insist that politics did not play into their analysis.

These officials say that there are powerful logical arguments for both the lab leak and the natural causes theories, but that there simply is no decisive piece of intelligence on either side of the issue.

To boost the natural origins theory, intelligence officers would like to find the animal that passed it to a human or find a bat carrying what was the likely ancestor of the coronavirus that causes Covid.

Similarly, to seal the lab leak, the intelligence community would like to find evidence that one of the labs in Wuhan was working on a progenitor virus that directly led to the epidemic.

Neither piece of evidence has been found.

But Mr. Ratcliffe has promised a more aggressive C.I.A., and it is possible that he will order more actions to penetrate the labs in Wuhan or the Chinese government in a search for information.

It will not be an easy secret to steal. The senior ranks of the Chinese government do not know, and do not want to know, American officials have said. So if there is intelligence, it is probably hidden in a place that is hard to get to.

Intelligence officials interviewed in recent weeks say it is possible that such a piece of evidence exists in a lab in China, at least in theory. But, they said, it is still more likely that the answers to questions surrounding the virus’s origins will come through a scientific breakthrough, not an intelligence revelation.

Under the Biden administration, the intelligence community leaned toward the theory that the virus came from the market. But officials readily admitted it was hardly a sure thing.

Five agencies, including the National Intelligence Council and the Defense Intelligence Agency, assessed that natural exposure most likely caused the epidemic. But they said that they had only low-confidence in their assessment.

Until now, two agencies, the F.B.I. and Department of Energy, thought a lab leak was more likely. But their theories are different. The F.B.I. believes the virus came from the Wuhan Institute of Virology. The Energy Department put its bet on another lab, the Wuhan Center for Disease Control.

Officials would not say if the C.I.A. believes one lab or the other was the more likely source of the virus.