The first known COVID-19 case was a market vendor in the Chinese city of Wuhan, not an accountant who appeared to have no link to the market but whose case contributed to speculation the virus could have leaked from a lab, a US researcher wrote in a commentary piece published on Thursday.

The origin of the SARS-CoV-2 virus that causes COVID-19 remains a mystery and a major source of tension between China and the United States.

The new analysis by the researcher does not provide a definitive answer to that question.

A joint study by China and the World Health Organisation (WHO) this year all but ruled out the theory that COVID-19 originated in a laboratory, saying that the most likely hypothesis was that it infected humans naturally, probably via the wildlife trade.

A WHO-led team of experts spent four weeks in and around the central city of Wuhan with Chinese scientists and said in a joint report in March that the SARS-CoV-2 virus had probably been transmitted from bats to humans through another animal but that further research was needed.

The accountant who was widely thought to be the first person with COVID-19, when interviewed, said his first COVID-19 symptoms appeared on December 16, several days later than initially reported, Michael Worobey, head of ecology and evolutionary biology at the University of Arizona, wrote in the commentary piece in the journal Science.

“His symptom onset came after multiple cases in workers at Huanan Market, making a female seafood vendor there the earliest known case, with illness onset 11 December,” Worobey said in the article.

He said most early symptomatic cases were linked to the market, specifically to the western section where raccoon dogs were caged, providing strong evidence of a live-animal market origin of the pandemic.

Worobey was among the 18 scientists who had written to Science in May calling for an investigation into whether the source of the virus could have been a laboratory accident.

The WHO proposed last month a new expert panel to investigate the source of the coronavirus.