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 – and why does the debate matter?
What is the lab-leak theory?
It’s a suspicion that the coronavirus may have escaped, accidentally or otherwise, from a laboratory in the central Chinese city of Wuhan where the virus was first recorded.
Its supporters point to the presence of a major biological research facility in the city. The Wuhan Institute of Virology (WIV) has been studying coronaviruses in bats for over a decade.
The laboratory is located just a few kilometres from the Huanan wet market where the first cluster of infections emerged in Wuhan.
Those who support the theory say it could have leaked from this facility and spread to the wet market.
Most argue it would have been an unaltered virus collected from the wild, rather than engineered.
The controversial theory first emerged early on in the pandemic, and was promoted by then-US President Donald Trump. Some even suggested it could have been engineered as a possible biological weapon.
While many in the media and politics dismissed these as conspiracy theories at the time, others called for more consideration of the possibility. Nevertheless, the idea resurfaced in recent weeks.
So why has it come up again?
Because reports swirling around the US media have raised fresh concerns over the lab-leak theory. And some scientists who were once sceptical of the idea have expressed fresh openness to it.
A classified US intelligence report – saying three researchers at the Wuhan laboratory were treated in hospital in November 2019, just before the virus began infecting humans in the city – began circulating in US media this week.
But it was reported the Biden administration had shut down a US state department investigation, set up by President Trump, into the lab-leak theory.
“That possibility certainly exists, and I am totally in favour of a full investigation of whether that could have happened,” Anthony Fauci, President Biden’s chief medical adviser, told the US senate committee on 11 May.
President Biden now says he asked for a report on the origins of Covid-19 after taking office, “including whether it emerged from human contact with an infected animal or from a laboratory accident”.
On Tuesday, Mr Trump sought to take credit for the renewed interest in an emailed statement to the New York Post. “To me it was obvious from the beginning but I was badly criticised, as usual,” he said. “Now they are all saying: ‘He was right.'”
What do scientists think?
The issue is still being hotly contested.
A World Health Organisation (WHO) investigation was supposed to get to the bottom of it, but many experts believed it produced more questions than answers.
team of WHO-appointed scientists flew to Wuhan earlier this year on a mission to investigate the source of the pandemic. After spending 12 days there, which included a visit to the laboratory, the team concluded the lab-leak theory was “extremely unlikely”.
But many have since questioned their findings.
A prominent group of scientists criticised the WHO report for not taking the lab-leak theory seriously enough – it was dismissed in a few pages of a several-hundred-page report.
“We must take hypotheses about both natural and laboratory spillovers seriously until we have sufficient data,” the scientists wrote in Science Magazine.
And there is growing consensus among experts that the laboratory leak should be looked at more closely.
Even the WHO’s own director-general, Dr Tedros Adhanom Ghebreyesus, has called for a new investigation, saying: “All hypotheses remain open and require further study.”
And Dr Fauci now says he’s “not convinced” the virus originated naturally. That’s a shift from a year ago, when he thought it most likely Covid had spread from animals to humans.
What does China make of this?
China has hit back at suggestions the virus may have escaped from a laboratory by calling it a smear, and it has suggested the coronavirus may have have entered the country in food shipments from another country.
The Chinese government points to new research published by one of its leading virologists into samples collected from bats in a remote abandoned mine.
Prof Shi Zhengli – often referred to as “China’s Batwoman” – a researcher at the Wuhan Institute, published a report last week revealing that her team had identified eight coronavirus strains found on bats in the mine in China in 2015. The paper says that coronaviruses from pangolins pose more of an immediate threat to human health than the ones her team found in the mine.
China’s state media have accused the US government and Western media of spreading rumours.
“The public opinion in the US has become extremely paranoid when it comes to the origin of the pandemic,” an editorial in the Communist Party-owned Global Times newspaper said.
Instead, the Chinese government has been pushing another theory: that the virus reached Wuhan on frozen meat from China or South-East Asia.
Is there another theory?
Yes, and it’s called the “natural origin” theory.
This argues the virus spread naturally from animals, without the involvement of any scientists or laboratories.
Supporters of the natural origin hypothesis say Covid-19 emerged in bats and then jumped to humans, most likely through another animal, or “intermediary host”.
That idea was backed by the WHO report, which said it was “likely to very likely” that Covid had made it to humans through an intermediate host.
This hypothesis was widely accepted at the start of the pandemic, but as time has worn on, scientists have not found a virus in either bats or another animal that matches the genetic make-up of Covid-19, casting doubt over the theory.
Why does this matter?
Given the massive human toll of the pandemic – which has now claimed the lives of 3.5 million people worldwide – most scientists think understanding how and where the virus originated is crucial to prevent it happening again.
If the “zoonotic” theory is proved correct, it could affect activities such as farming and wildlife exploitation. In Denmark, fears about the spread of the virus through mink farming led to millions of mink being culled.
But there are also big implications for scientific research and international trade if theories related to a laboratory leak or frozen food chains are confirmed.
And confirmation of a leak may also affect how the world views China, which has already been accused of hiding crucial early information about the pandemic, and place further strain on US-China relations.
“From day one China has been engaged in a massive cover-up,” Jamie Metzl, a fellow at the Washington-based Atlantic Council who has been pushing for the lab-leak theory to be looked into, told the BBC.
“As the evidence for the lab-leak hypothesis grows, we should be demanding the full investigation of all origin hypotheses that’s required.”
But others cautioned against pointing the finger at China too quickly.
“We do need to be a bit patient but we also need to be diplomatic. We can’t do this without support from China. It needs to be a no-blame environment,” Prof Dale Fisher, of Singapore’s National University Hospital, told the BBC.