A World Health Organization (WHO) team of 10 international scientists arrived at the Chinese city of Wuhan to investigate the origins of the novel coronavirus (Covid-19).
In the early days of the virus, it was traced to a so-called "wet market" in Wuhan, Hubei province, and it was suggested that this was where it made the leap from animals to humans.
But experts now believe it may simply have been amplified there.
"It's really not about finding a guilty country," Fabian Leendertz of Germany's Robert Koch Institute said. "It's about trying to understand what happened and then see if, based on those data, we can try to reduce the risk in the future."
Beijing has been reluctant to agree to an independent inquiry and it has taken many months of negotiations for the WHO to be allowed access to the city.
The virus is thought to have come from a market in the city selling animals. But the search for the source has led to tensions, notably with the US.