Beijing has never fully co-operated with international investigators, impeding the search for a definitive answer. But scientists continue their quest to establish the causes of a global health crisis that killed millions of people, caused trillions of dollars of economic damage and increased fears over the risks of virus research.

The intelligence agency says it has a “low confidence” in its new finding, but this is further than it has ever gone in pinpointing the origin of a virus that killed millions worldwide.