White House official John Kirby, standing at the podium where Donald Trump once railed against the “China virus” and praised the healing powers of bleach, faced questions on Monday about the origins of Covid-19. He had no choice but humility. “There is not a consensus right now in the US government about exactly how Covid started,” Kirby admitted. “There is just not an intelligence community consensus.”

The renewed interest in a genuine scientific mystery followed a report in the Wall Street Journal that the US Department of Energy had determined the coronavirus most likely leaked by accident from a Chinese laboratory.

This startling assessment appeared to have a solid foundation: according to the Washington Post, it was based on an analysis by experts from the national laboratory complex, including the “Z-Division”, known for carrying out some of the American government’s most secretive and technically challenging investigations of security threats from adversaries such as China and Russia.

But the claim was not officially confirmed by the energy department or Kirby, and it came with a caveat: the department had “low confidence” in its assessment, which was provided to the White House and certain members of Congress, the Journal said.

Even so, gleeful Republicans seized on the findings to claim vindication in their pursuit of the lab leak theory, triggering a fresh round of toxic debate in Washington and on social media.

Opponents say there is still no hard evidence for a lab leak, as many scientists still believe the virus most probably came from animals, mutated and jumped into people. They note that the loudest champions of the lab leak hypothesis are often also trafficking in rightwing conspiracy theories, for example about the top infectious disease expert Anthony Fauci.

But the two do not necessarily go hand in hand. Some scientists and other observers argue that the lab leak theory cannot be ruled out and should be kept separate from the racist propaganda that often accompanies it. It demands careful investigation, not peremptory dismissal or acceptance, they contend.

It is the latest chapter in a long fight over the origin of a virus that has caused close to 7m deaths worldwide, clouding efforts to pursue a neutral, fact-based inquiry. In its loud opinions, blue v red certainties and lack of nuance, the melee echoes clashes over pandemic lockdowns, masks and vaccines, as well as the investigation into Trump’s alleged collusion with Russia.

Bill Galston, a former policy adviser to Bill Clinton, said: “Isn’t this just like everything else in American politics, where a partisan position on one side invites a partisan response by the other? There’s a lot of what might be called reactive thinking going on because of the high degree of polarisation and the high stakes. Charges without foundation invite responses without foundation.”

Calling for public hearings into the matter, Galston, a senior fellow at the Brookings Institution thinktank in Washington, warned: “If this isn’t lifted out of the crucible of political debate right now, it’ll just get worse and worse.”

Studies by experts around the world have indicated that Covid-19 most likely emerged from a live animal market in Wuhan, China. The hypothesis that it originated from an accidental lab leak was initially dismissed by most public health experts and government officials.

In February 2020, the Lancet medical journal published a statement that rejected the lab leak theory, signed by 27 scientists and expressing “solidarity with all scientists and health professionals in China”. It asserted: “We stand together to strongly condemn conspiracy theories suggesting that Covid-19 does not have a natural origin.” (The journal later disclosed that the organiser of the letter had links to the Wuhan lab at the center of the controversy.)

That the lab leak theory was being pushed by Trump, who long played down the virus and used xenophobic language such as “China virus”, and his secretary of state, Mike Pompeo, may have contributed to the instinctive eagerness of some to dismiss the hypothesis – and to ostracise scientists who dared question the mainstream orthodoxy.

“From the start, the lab leak theory was never properly framed and parsed,” David Relman, a microbiology and immunology professor at Stanford University in Palo Alto, California, wrote in an email. “The hypothesis of a lab-associated origin became synonymous with deliberate efforts to engineer viruses and malevolent intent, and this has not been helpful. The emotions, assumptions about motives, obstructionism by the Chinese government, and poor scrutiny of the evidence have only made things worse.”

Jackson Lears, a history professor at Rutgers University in New Brunswick, New Jersey, echoed this view: “People who consider themselves Democratic party sympathisers and liberals uncritically arrayed themselves against this. It was a kind of a lockstep reaction against Trump, as in so many matters.”

The lab leak hypothesis did begin to receive scrutiny after Joe Biden ordered an intelligence investigation in May 2021. The 90-day review was intended to push US intelligence agencies to collect more information and review what they already had.

But the review proved inconclusive. A report summary said four members of the US intelligence community believed with low confidence that the virus was first transmitted from an animal to a human, and a fifth believed with moderate confidence that the first human infection was linked to a lab. Two agencies – including the CIA – remain undecided.

Without the equivalent of a special counsel delivering a final report, the White House is left in a fog of uncertainty that satisfies no one. Lears commented: “There should have been a more carefully orchestrated investigation, more centralised, more high profile, with more legitimacy. Splitting it up and into many agencies is a way of defanging the whole situation.”

Others agree that the multiple investigations give Biden a political headache, especially at a moment of rising tensions with China over trade, Taiwan, Ukraine and a recent spy balloon shot down after transiting US airspace.

Laurie Garrett, a columnist at Foreign Policy magazine who spent time in China during the Sars outbreak, witnessing how animal markets operated, said: “The president said, ‘I want the relevant agencies in the government to take a close look at this.’ Well, every agency has its own prism, its own skill set.

“In Britain if you asked the Home Office, MI5, the Metropolitan Police, the London School of Hygiene and Tropical Medicine and the British Medical Association to take a look, you would get seven different answers and that’s the situation that the Biden administration has created for itself. By trying to appease all the screaming and cut the rightwing Republicans off at the knees on this, they’ve essentially opened up a Pandora’s box because every single agency is going to have a different way of looking at the problem.”

Many scientists, including Fauci, who until December served as Biden’s chief medical adviser, say they still believe the virus most likely emerged in nature and jumped from animals to humans, an established phenomenon known as a spillover event. But the reports of dissent in the intelligence community will give enough oxygen to those with doubts, good faith or otherwise.

Jeremy Konyndyk, president of Refugees International and formerly USAid’s lead official for Covid-19, likens it to a Rorschach test. He said: “The priors that you come in with are going to shape a lot of how you interpret the evidence, because ultimately, the evidence may suggest one way or another, but it’s not definitive one way or another.

“If you want to craft a narrative that justifies the lab leak theory, you can do so. If you want to craft a narrative that justifies a natural origin, natural spillover, market amplification theory, you can do so. There’s not enough on either side to definitively rule in or out either.”

But that does not make them equally plausible, Konyndyk added. “The preponderance of evidence strongly points to a natural spillover, occurring at and certainly amplified at the market.” Konyndyk noted how online debate about the issue has become toxic, with proponents of the lab leak making death threats to scientists. “There’s been some really irresponsible behaviour and they’re not trying to turn the temperature down.

“That has prompted in turn very strong views from some of the more vocal folks who believe in the natural origin theory because they’re getting attacked on Twitter with a larger and larger army of trolls. It’s just gotten crazy.”

Earlier this month, Republicans in the House of Representatives issued letters to current and former Biden administration officials for documents and testimony, exploring the hypothesis of a lab leak. Congressman Brad Wenstrup, chair of the House oversight panel’s virus subcommittee, has accused US intelligence of withholding key facts about its investigation.

Garrett, author of The Coming Plague: Newly Emerging Diseases in a World Out of Balance, added: “My concern about where we are right now with this whole Wuhan origins question is that several very serious, real issues are getting conflated and they’re being manipulated for political purposes by people who don’t understand the issues at all and don’t care.

“We’re not hearing in these congressional hearings this is what we should do to strengthen the chemical, biological warfare agreements and make lab research safe in the world. Nobody’s saying that. They couldn’t care less. That’s not their agenda. Their agenda is to tear down a man who was seen on camera in a live press conference putting his hand over his face and shaking his head as President Trump said, ‘Maybe bleach can cure Covid.’”