Nearly a year and a half since Covid-19 was detected in the Chinese city of Wuhan, the question of how the virus first emerged remains a mystery.
But in recent weeks the controversial claim that the pandemic might have leaked from a Chinese laboratory – once dismissed by many as a fringe conspiracy theory – has been gaining traction.
Now, US President Joe Biden has announced an urgent investigation that will look into the theory as a possible origin of the disease.
China’s foreign ministry accused the US of "political manipulation and blame shifting", rejecting any link between Covid-19 and a virus research lab in the Chinese city of Wuhan.
Ros Atkins explores what the US investigations are covering.
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