The origins of Covid-19 can’t be conclusively tied to the market in Wuhan where some of the first known cases appeared, according to Chinese researchers, further extending controversy around data collected at the start of the pandemic that had long remained out of view.

Findings from the samples taken more than three years ago have generated debate that intensified last month after the Chinese researchers briefly posted the data in an open-access genomics database. An outside group of academics who ran their own analysis concluded it was the strongest data yet backing the theory that the virus spilled over from animals to humans at the market. In a report published in the journal Nature, the Chinese researchers disagreed.

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