Why the Wuhan lab-leak theory is being taken seriously

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.

So what do we know about the competing theories...