They returned to their state-of-the-art laboratory in central China with tubes and vials containing known killers — pathogens associated with diseases that are deadly in humans — and also a few surprises. On multiple occasions, their takings included exotic coronaviruses previously unknown to science.

The highlights of the Wuhan researchers’ work on bat viruses are spelled out in more than 40 published studies and academic papers that describe a sprawling, ambitious effort to document the connection between bats and recent disease outbreaks in China. The experiments were intended to illuminate how dangerous pathogens sometimes jump from animal hosts to humans. But experts say the research also carried an implicit risk: the possibility that the lab itself could facilitate the spread of the very diseases the scientists were trying to prevent.

On Thursday, the U.S. intelligence community released an assessment formally concluding that the virus behind the coronavirus pandemic originated in China. While asserting that the pathogen was not man-made or genetically altered, the statement pointedly declined to rule out the possibility that the virus had escaped from the complex of laboratories in Wuhan that has been at the forefront of global research into bat-borne viruses linked to multiple epidemics over the past decade.

“The IC will continue to rigorously examine emerging information and intelligence to determine whether the outbreak began through contact with infected animals or if it was the result of an accident at a laboratory in Wuhan,” the Office of the Director of National Intelligence said, using a common initialism for the U.S. intelligence community.

Yet, despite the intense scrutiny, the novel coronavirus’s origins remain as murky now as they did when the first cases emerged in China five months ago. While intelligence analysts and many scientists see the lab-as-origin theory as technically possible, no direct evidence has emerged suggesting that the coronavirus escaped from Wuhan’s research facilities. Many scientists argue that the evidence tilts firmly toward a natural transmission: a still-unknown interaction in late fall that allowed the virus to jump from a bat or another animal to a human.

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“It’s far more likely that Mother Nature is just a step ahead of us and has created a novel pathogen, now able to move quite effectively from human to human,” said Jason Rao, a biosecurity specialist, former senior policy adviser to President Barack Obama and executive director of Health Security Partners, a D.C.-based nonprofit organization focused on global biological threat reduction.

Chinese officials and scientists have strenuously denied any connection between the coronavirus outbreak and its showcase research center, which includes a high-security facility known as the Wuhan Institute of Virology. The Wuhan team leader, renowned virologist Shi Zhengli, contends that the institute never possessed the SARS-cov-2 virus that triggered the pandemic and has infected more than 3 million people worldwide. In a social media post, Shi said she would “bet my life” that the outbreak had “nothing to do with the lab.”

At the same time, scrutiny of the lab’s research has underscored what biosecurity experts say are significant risks inherent in the kinds of research the Chinese scientists were conducting. Academic studies examined by The Washington Post document scores of encounters with animals that are known hosts to deadly viruses, including strains closely related to the pathogen behind the coronavirus pandemic. While the scientists wore gloves and masks and took other protective measures, U.S. experts who reviewed the experiments say the precautions would not necessarily protect the researchers from harmful exposures, in caves or in the lab.

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The risks were not limited to interactions with animals. Dozens of routine studies required extracting viruses from bat feces and growing them in batches for use in a wide array of experiments. For some projects the researchers spliced genetic material from different coronaviruses to create chimeras that could more easily infect human cells for laboratory experiments.

The research filled in critical gaps in scientists’ knowledge about deadly viruses and prompted Chinese scientists to issue repeated warnings about the possibility of a new SARS-like disease making the leap from bats to humans. But with each experiment came opportunities for an accidental exposure to dangerous pathogens, experts say. Indeed, such accidents occur dozens of times each year in high-security laboratories around the world, including in the United States.

The National Institutes of Health, the Defense Department and other U.S. government agencies have spent millions of dollars in recent years to fund research by American scientists into coronaviruses in bats, federal records show. Some of those scientists have worked with colleagues at the lab in Wuhan.

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“Even if a lab is mechanically safe, you can’t rule out human error,” said Lynn Klotz, a senior science fellow at the Center for Arms Control and Non-Proliferation, a Washington nonprofit group, and author of a comprehensive study of lab mishaps. “Accidents happen, and more than 70 percent of the time it’s due to the humans involved.”

Records of accidents in U.S. labs reveal multiple inadvertent infections and exposures to lethal microbes, including the pathogens linked to anthrax, Ebola and the plague. While no comparable records are available for Chinese labs, a Chinese scientific paper last year described widespread systemic deficiencies with training and monitoring of high-security laboratories where disease-causing pathogens are studied.

“Maintenance cost is generally neglected; several high-level BSLs [biological safety level labs] have insufficient operating funds for routine, yet vital processes,” said the paper by Yuan Zhiming, a chief scientist at Wuhan, published in the Journal of Biosafety and Biosecurity. Most laboratories “lack specialized biosafety managers and engineers,” he wrote.

While the source of the outbreak ultimately may be unknowable, the claim that the laboratory could not have been involved in the virus’s release “is not credible,” said Richard Ebright, a professor of chemistry and chemical biology at Rutgers University.

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David Relman, a Stanford University professor of microbiology, said the outbreak at a minimum underscores the need for more stringent standards and comprehensive monitoring of research involving pathogens with the ability to inflict widespread harm on human health and economies.

“There are far too many examples of lab accidents. Our own CDC and everyone else has had accidents, even with very dangerous agents,” Relman said. “There is simply no way around it, since humans are flawed — inconsistent, distractible — creatures.”

'We just don't know'

But while an accidental release may have been possible, no proof of such of an event has emerged. The closest relative to the coronavirus that causes covid-19 known to have existed at Wuhan was still a distant relative, scientists say. In March, a landmark study of the virus’s origins in the journal Nature Medicine played down the possibility of an accident, saying that “we do not believe that any type of laboratory-based scenario is plausible.”

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Scientists who worked closely with Shi and other Wuhan scientists described the researchers as particularly diligent and careful.

Maureen Miller, an infectious disease epidemiologist who worked with Shi as part of a U.S.-funded viral research program, dismissed the lab-as-origin theory as an “absolute conspiracy theory” and referred to Shi as “brilliant.”

“She is a rigorous scientist,” Miller said. “She is very, very committed to preventing the kind of scenario that is happening right now.”

Concerns about unwarranted scapegoating of Shi and other Chinese scientists have increased following reports that the Trump administration has sought to pressure U.S. intelligence agencies to search for proof of a link between the Wuhan lab and the outbreak. The New York Times reported Thursday that some analysts fear the administration will seek to distort assessments about the virus as a means of blaming China for a pandemic that has already sickened more than 1 million Americans and killed more than 60,000.

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On Thursday, President Trump suggested at a briefing that he had evidence of a connection between the Wuhan lab and the pandemic. “Yes, I have,” Trump said when asked whether he’s seen anything that makes him believe that lab workers were responsible. He did not elaborate.

Policymakers, during private intelligence briefings, have been told that Chinese officials tried to obscure the severity of the virus in its early days, but intelligence agencies saw no direct evidence that China was attempting to cover up a lab accident, according to one U.S. intelligence official familiar with the matter who spoke on the condition of anonymity to discuss administration deliberations.

“What we know is it’s naturally occurring,” a second intelligence official said. “We know that it came from Wuhan. There’s been speculation: Did it come from a market? Did it come from a lab? We just don’t know.’

'This won't be the last event'

The Wuhan institute’s intensive focus on bats and their diseases began 25 years ago, when researchers started investigating the origins of the respiratory disease SARS, another viral illness blamed for the deaths of about 1,000 people in the early 2000s. Chinese scientists eventually traced the virus to horseshoe bats living in caves in the country’s southern Yunnan province. Subsequent studies confirmed that bats are natural reservoirs for numerous zoonotic diseases, and over the years Wuhan’s scientists embarked on scores of experiments to study them, sometimes in collaboration with researchers from the United States, Australia and other countries.

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To minimize risk of accidental infection during field work, the researchers wore goggles, tear-resistant gloves and N95 masks similar to the ones used by medical workers in hospitals, the research papers show. But the protective gear, while helpful, would not necessarily have shielded the workers from being scratched or bitten by horseshoe bats, say U.S. scientists who have participated in research involving live animals. Moreover, N95 masks are inadequate for blocking all viruses, even when used properly, the scientists said.

“Whether the staff are interacting with bats in the wild or in the lab, they are routinely putting themselves at risk of infection,” said one U.S. scientist with extensive experience in federal government labs that study human pathogens using live animals. The scientist spoke on the condition of anonymity because he was not authorized to speak publicly on foreign nations’ lab practices.

More controversial was the Wuhan institute’s 2015 research into creating a chimera, the hybrid virus that combined elements from two bat-borne coronaviruses, including one that causes SARS. The mutated virus that resulted was more easily able to infect human cells, making it more useful for lab experiments. Such “gain of function” experiments — which enhance a pathogen’s natural traits — have been a source of controversy in the West because of the potential for harm if an altered strain escapes the confinement of the lab, experts say.

“No lab worker goes to work planning to acquire an infection,” the U.S. scientist said. “My concern is that this won’t be the last event of this nature if the entire world doesn’t adopt better safety and transparency in laboratory practices.”