International scientists who examined previously unavailable genetic data from samples collected at a market close to where the first human cases of Covid-19 were detected in China said they found suggestions the pandemic originated from animals, not a lab.

Other experts have not yet verified their analysis, which also has not appeared so far in a peer-reviewed journal. How the coronavirus first started sickening people remains uncertain.

“These data do not provide a definitive answer to how the pandemic began, but every piece of data is important to moving us closer to that answer,” WHO Director-General Tedros Adhanom Ghebreyesus at a Friday press briefing.

He also criticized China for not sharing the genetic information earlier, adding that “this data could have and should have been shared three years ago.”

The samples were collected from surfaces at the Huanan seafood market in Wuhan after the first human cases of Covid-19 were found in late 2019.

Tedros said the genetic sequences were uploaded to the world’s biggest public virus database in late January by scientists at the Chinese Center for Disease Control and Prevention; the data have since been removed from the database.

A French biologist spotted the information by chance while scouring the database and shared it with a group of scientists based outside China and looking into the origins of the coronavirus.

Genetic sequencing data showed that some of the samples, which were known to be positive for the coronavirus, also contained genetic material from raccoon dogs, indicating the animals may have been infected by the virus, according to the scientists. Their analysis was first reported in The Atlantic.

“There’s a good chance that the animals that deposited that DNA also deposited the virus,” said Stephen Goldstein, a virologist at the University of Utah who was involved in analyzing the data. “If you were to go and do environmental sampling in the aftermath of a zoonotic spillover event … this is basically exactly what you would expect to find.”

Ray Yip, an epidemiologist and founding member of the U.S. Centers for Disease Control office in China, said that even though the new findings weren’t definitive, they were significant.

“The market environmental sampling data published by China CDC is by far the strongest evidence to support animal origins,” Yip told the AP in an email. He was not connected to the new analysis.

Scientists have been looking for the origins of the Covid-19 pandemic since the virus first emerged, but that search has been complicated by factors including the massive surge of human infections in the pandemic’s first two years and an increasingly bitter political dispute.

It took virus experts more than a dozen years to pinpoint the animal origin of SARS, a related virus.

The researchers say their analysis is the first solid indication that there may have been wildlife infected with the coronavirus at the market. Some of the samples with raccoon dog DNA were collected from a stall that tested positive for Covid-19 and was known to be involved in the wildlife trade, Goldstein said.

But it is also possible that humans might have first brought the virus to the market and infected the raccoon dogs, or that infected humans happened to leave traces of the virus near the animals.

After scientists in the group contacted the China CDC, they say, the sequences were pulled from the global virus database. Researchers are puzzled as to why data on the samples collected over three years ago wasn’t made public sooner.

Earlier this week, some of the scientists presented their findings to an advisory group the World Health Organization has tasked with investigating Covid's origins, Goldstein confirmed.

Mark Woolhouse, an infectious diseases expert at the University of Edinburgh, said it will be crucial to see how the genetic sequences from the raccoon dogs match up to what’s known about the historic evolution of the Covid-19 virus.

He said that if the analysis shows the animal viruses have earlier origins than the ones that infected people, “that’s probably as good evidence as we can expect to get that this was a spillover event in the market.”

After a weeks-long visit to China to study the pandemic’s origins, WHO released a report in 2021 concluding that Covid most probably jumped into humans from animals, dismissing the possibility of a lab origin as “extremely unlikely.”

But the U.N. health agency backtracked the following year, saying “key pieces of data” were still missing.

In recent months, WHO director Tedros has said all hypotheses remained on the table, while he and senior officials pleaded with China to share more data about their Covid-19 research.

The China CDC scientists who previously analyzed the samples published a paper as a preprint in February. Their analysis suggested that humans brought the virus to the market, not animals, implying that the virus originated elsewhere.

The paper did not mention that animal genetic material was found in samples that tested positive for Covid-19, and the authors didn’t upload the raw data until March. Gao Fu, the former head of the China CDC and lead author of the paper, didn’t immediately respond to an email requesting comment.

Wuhan, the Chinese city where Covid-19 was first detected, is home to several labs involved in collecting and studying coronaviruses, fueling theories that the virus may have leaked from one.