On their fourth full day of work in China, the World Health Organization mission investigating the origins of the coronavirus pandemic has visited the Wuhan Center for Disease Control, suspected of having been a key site in the early history of Covid-19.
The team's visit to the centre, which played an early part in managing the outbreak, came against a background of tight Chinese controls on access to information about the virus.
The World Health Organization investigators arrived in Wuhan, the Hubei provincial capital, last month to look for clues about the origins of the coronavirus. They have visited hospitals that treated many of the earliest patients and a seafood market where cases of infection with the then-unknown virus emerged in December 2019.
The mission is politically charged, as China seeks to avoid blame for alleged errors in its early response to the outbreak.
Years of research
A single visit by scientists is unlikely to confirm the virus's origins –pinning down an outbreak's animal reservoir is typically an endeavour requiring years of research, including taking animal samples, genetic analysis and epidemiological studies.
One possibility is that a wildlife poacher might have passed the virus to traders who carried it to Wuhan.
The Chinese government has promoted theories, with little evidence, that the outbreak might have started with imports of frozen seafood tainted with the virus, a notion roundly rejected by international scientists and agencies.
A possible focus for investigators is the Wuhan Institute of Virology in the city where the outbreak began.
One of China's top virus research labs, it built an archive of genetic information about bat coronaviruses after the 2003 outbreak of Severe Acute Respiratory Syndrome.