But as Chinese government officials sought to deflect blame, and as the Trump administration and, later, intelligence officials in the Biden administration, pointed to the possibility of a virology research lab in Wuhan, the picture has become muddled.

Worobey, who specializes in tracing the genetic evolution of viruses, has found considerable evidence that the virus arose in an animal, and did not start circulating until the end of 2019. He has published multiple studies on the probable origins of the virus. But he said he wanted to check his own theories against the real-life evidence from what was known about actual people with documented infections.

So he dove into the known cases. He says what he found strengthens the theory that the virus originated in animals sold in the market — much like the first SARS outbreak in 2002-2004 that infected 8,000 people before it was stopped.

One case especially stood out — that of a 41-year-old accountant who allegedly got sick on December 8, 2019 and who had no connection to the market. The case has been cited as evidence the pandemic must not have started at the market. Worobey found records that showed the man didn’t become ill with Covid-19 until later in December and that his December 8 problem was related to his teeth.

That would make a seafood vendor who worked at the market and who got sick December 11, 2019, the earliest documented case, Worobey said.

Other research helped Worobey come up with a map of the earliest cases that clusters them all around the market.

“That so many of the more than 100 COVID-19 cases from December with no identified epidemiologic link to Huanan Market nonetheless lived in its direct vicinity is notable and provides compelling evidence that community transmission started at the market,” he wrote.

“Big red flashing arrow”

“It tells us that there’s a big red flashing arrow pointing at Huanan Market as the most likely place that the pandemic started,” Worobey told CNN. “The virus didn’t come from some other part of Wuhan and then get to Huanan market. The evidence speaks really quite strongly to the virus starting at the market and then leaking into the neighborhoods around the market.”

The World Health Organization made its own investigation into the origins of the pandemic, and said animals were the most likely origin of the pandemic. But WHO also said its conclusions were not definite and asked the Chinese government to provide more information and access.

This information might not ever come to light, Worobey said. The Chinese government cleared the Huanan market of all animals and disinfected it when it became clear it was associated with an outbreak of infectious disease in January of 2020– removing the risk of more spread, but also destroying crucial evidence.

“I wouldn’t call this conclusive evidence, but I would call this pretty darn strong evidence,” Worobey said.

The journal Science subjected Worobey’s research to outside scrutiny before publishing it. And Peter Daszak, president of EcoHealth Alliance and one of WHO’s investigators, said he thinks it holds up.

“I was really impressed with the detective work he’s put in. Everything he says about the Dec 8th case fits with what we experienced in Wuhan on the WHO trip — there were a cluster of early cases coming into hospitals in late December and clinicians worked back to the presumed date of onset,” Daszak told CNN by email.

“They just made a mistake with this person because he likely visited hospital for another reason. This puts the first known case as a Huanan market worker, not the accountant who lived near one of the Wuhan lab campuses,” Daszak added. “This is now adding to around 10 pieces of other scientific evidence that I’ve seen since the end of our WHO work, all of which point towards an origin through the wildlife farms and markets. No single piece of evidence is totally conclusive, but when you lay them all out, it really tips the balance towards the ‘natural’ origin.”

Worobey joined a group of scientists who signed a letter in Science in May saying the theory that the coronavirus leaked from a lab needs thorough investigation.

“I co-signed that letter in Science suggesting that the lab leak needs to be investigated, which I still believe and that it should be,” Worobey told CNN. “But in the meantime, holy smokes — is there a lot of evidence against it and in favor of natural origin.”