An analysis by experts at the US National Laboratory Complex — including members of a legendary team known as Z-Division — prompted the Department of Energy to change its mind earlier this year about the likely cause of the coronavirus outbreak in 2019, the officials said. While initially doubtful about the origins of covid-19, energy officials concluded as part of a new government-wide intelligence assessment that a lab accident most likely triggered the world’s worst pandemic in a century.

But other intelligence agencies involved in the classified update — completed in recent weeks and kept secret — were divided on the question of the origins of Covid-19, with most still insisting that a natural, evolutionary “spill over” of animals caused the most likely was. explanation. Even the Energy Department’s analysis was carefully shielded, as the officials expressed only “little confidence” in their conclusion, according to US officials who spoke on condition of anonymity to describe a classified report.

The overall picture — that there has not yet been a definitive conclusion about the origin of the virus — has not changed since the publication of an earlier version of the report by the Biden administration in 2021, the officials said.

“The bottom line remains the same: In fact, no one really knows,” said one of the officials.

Still, news that Energy had changed its mind reignited what was already an intense debate on social media and in Washington, where members of Congress are preparing for hearings — some as early as Tuesday — investigating the circumstances behind the outbreak.

“To prevent the next pandemic, we need to know how it started,” said Rep. Mike Gallagher (R-Wis.), a member of the House Permanent Select Committee on Intelligence, in response to news of the updated assessment, first reported the Wall Street Journal Sunday. “The administration must act with a sense of urgency and use every tool at its disposal to ensure we understand the origins of COVID-19.”

US officials confirmed that an updated assessment of the origins of COVID-19 had been completed this year, saying the document was based on new data as well as new analysis by experts from eight intelligence agencies and the National Intelligence Council.

But the agencies are united, the official said, in the view that the virus was not developed as a biological weapon.

“’Lab’ does not equate to ‘man-made,’ the official said, noting that lab workers could have collected the virus in nature and stored it in the lab from which it escaped. “Even if it was a leak from a lab,” the person added, “intelligence analysts still think it would be a naturally occurring virus.”

Of the nine intelligence agencies involved in the review, only the FBI had previously concluded with “moderate” confidence that Covid-19 began with a lab accident. The Energy Department was the only agency to change its mind, while the CIA and one other agency remained undecided, not having enough compelling evidence to support one conclusion over another, officials said.

But even with low confidence, the Energy Department’s analysis weighs in. For the assessment, the department called on the expertise of a team composed of the US National Laboratory Complex, which employs tens of thousands of scientists representing many technical specialties, from physics and data analytics to genomics and molecular biology.

The laboratories were established as part of the US nuclear weapons program and operate largely in the secret domain. The department’s cadre of technical experts includes members of the Energy Department’s Z Division, which has been involved in covert investigations into nuclear, chemical, and biological weapons threats by U.S. adversaries, including China and Russia, since the 1960s.

The Department of Energy is “a technical organization with tens of thousands of scientists,” said a former energy official. “It’s more than just physics. It’s chemical and biological expertise. And they have a unique opportunity to look at intelligence from the technical side.”

Both the Energy Department and the Office of the Director of National Intelligence declined to comment on the revised assessment. It was unclear exactly what led officials at Energy to see a lab leak as the more likely explanation for how covid-19 started.

None of the leading theories — a natural spillover or a lab leak — has been conclusively validated, in part because of China’s refusal to allow independent researchers access to environmental samples and other raw data from the first weeks of the outbreak.

Many scientists — and, at least for now, the majority of U.S. intelligence agencies — favor the spillover hypothesis, which holds that the virus jumped from bats to humans, perhaps in a Chinese market, and presumably after passing through a third strain that had come to house what became known as the SARS-CoV-2 virus. But three years after the outbreak began, the search for the elusive “carrier species” has turned up no solid leads. The bats that naturally harbor viruses closely related to SARS-CoV-2 are native to Southeast Asia and southern China, about 1,000 miles from Wuhan, where the first cases of Covid-19 were reported.

Likewise, no hard evidence of a lab leak has emerged. Proponents of the leak theory note that the outbreak began in a city that happens to be the world’s leading center for coronavirus research. China has had previous lab accidents, including a 2004 incident where lab workers were inadvertently exposed to the original SARS virus and then spread the pathogen outside the lab, causing multiple illnesses and at least one death, according to a World Health Organization inquiry.

China has repeatedly denied that there was an accident. On Monday, Beijing denounced the new report linking Chinese laboratories to the pandemic, with Chinese foreign ministry spokeswoman Mao Ning demanding that the United States “stop defaming China.”

“Covid tracing is a scientific problem that should not be politicized,” she said.

The Biden administration on Monday emphasized the inconclusive nature of the evidence to date. National Security Council Coordinator for Strategic Communications Speaking at a White House briefing, John Kirby said the new intelligence assessment was part of an ongoing effort by “the entire government” to investigate how COVID-19 started, though he acknowledged that firm conclusions have remained elusive.

“There is currently no consensus in the US government on exactly how covid started,” Kirby told reporters. “That work is ongoing, but the president believes it is very important that we continue that work and that we do the best we can to find out how it started so that we can better prevent a future pandemic.”

National Intelligence Director Avril Haines will testify next week at a Senate hearing on global threats and will likely be asked to handle the case. The selected House subcommittee on the coronavirus pandemic was scheduled to hold a roundtable on early COVID-19 policy decisions on Tuesday.