In the video, the researchers scale the cavern wall, their headlamps ghostly blue

“If our skin is exposed, it can easily come in contact with bat excrement and contaminated matter, which means this is quite risky,” says Tian Junhua, one of the bat hunters.

“We have to live for several days in the cave . . .” he continues, as the soundtrack amps up the drama. “There’s no cellphone signal, no supplies. It’s truly scary.”

The video was released by national science authorities and Chinese state broadcaster CCTV on Dec. 10, 2019, and circulated on social media. It’s a high-quality production, designed to promote China’s world-leading viral research. Aired around the time Wuhan residents began turning up at hospitals with mysterious respiratory ailments, it also offers a rare glimpse of field conditions on the eve of the pandemic.

Tian and his team from the Wuhan Centre for Disease Control and Prevention are filmed catching horseshoe and pipistrelle bats and collecting samples of guano, in search of new bat-borne diseases and the basis of new vaccines. Tian talks about the need for caution. “It is while discovering new viruses that we are most at risk of infection,” he says, though he is shown handling sample vials without wearing full protective gear.

The video is perhaps even more notable for what it doesn’t reveal. Nothing is known outside China about the science gleaned from that expedition by the Wuhan CDC — the same agency that oversaw China’s early pandemic response. The team has not disclosed what viruses, if any, it found in the cave, or even when the mission took place. According to a World Health Organization report released in March, the Wuhan CDC denied any storage or laboratory activities involving bat viruses before the coronavirus outbreak — a stance hard to reconcile with Tian’s boasts in the video about having visited dozens of bat caves and studied 300 types of virus vectors.

Tian has not spoken publicly for more than a year.

The silencing of scientists, the blanket denials, the careful guarding of raw data and biological samples — these elements have been emblematic of the approach by Chinese authorities at every stage of the Coronavirus outbreak. And they continue to obstruct the world’s ability to get answers.

There is no direct evidence linking Tian’s team, or a rival group of bat-disease specialists at the Wuhan Institute of Virology, to the coronavirus outbreak. Nor is there more than circumstantial evidence to support any theory of the pandemic’s origin. Many scientists say the most likely path is that the virus spread in nature and jumped from animals to humans. But that belief is largely based on how other coronaviruses have originated, not what is known about this case.

The lack of clarity is not in itself alarming at this point in an investigation of virus origins — in the case of severe acute respiratory syndrome (SARS), scientists were fairly quick to identify civet cats as the intermediate host, though it took years to find the bat population that harboured the building blocks of the virus. But the WHO chief, the Biden administration, other governments and scientists around the world have rebuked China for not making this investigation any easier.

Last week, President Biden directed U.S. intelligence agencies to redouble efforts to determine the pandemic’s origin, including whether it could have emerged “from a laboratory accident.”

China’s Foreign Ministry protested, with a spokesman saying Monday that Beijing supported scientific inquiry on the question but not “an intelligence-led investigation stoking confrontation.”

Chinese authorities weren’t much more receptive of the international team commissioned by the WHO. Negotiations over the arrangements delayed the team from getting to Wuhan until more than a year after doctors first raised concerns there. Once on the ground, the international experts were given limited access. They visited the market linked to early coronavirus cases — but it had been shut for a year and its contents long ago removed. Their visit to the Wuhan Institute of Virology lasted three hours. In general, they had to satisfy themselves with data that was in large part collected by Chinese scientists before the trip.

In the absence of crucial evidence of how the new coronavirus began comes many theories — one is that the virus accidentally escaped from a lab in Wuhan, China. The result was a report that didn’t significantly advance the world’s understanding of how the pandemic came about.

The report lent credence to China’s preferred theory that the virus could have come from overseas, possibly via frozen food imports — though Beijing has presented little support for that. On the question of a possible lab leak, the report concluded that pathway was “extremely unlikely.”

Analysts have accused China of inappropriately influencing the team’s conclusions.

For Beijing, there are huge political risks if it loses control of the narrative, said Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations.

“They want to portray China as successful at containing the spread, at being so cooperative and sharing information,” he said. “This lab-escape theory, which indicates that China was the cause of the problem, makes the official narrative unreliable.”