Speculation has swirled since the start of the pandemic that Covid-19 may have emerged from a laboratory in the Chinese city of Wuhan. World Health Organization inspectors visited the lab on Wednesday.
The WHO experts have already visited a number of key sites in Wuhan, where Covid-19 was first detected in December 2019.
But the trip to the Wuhan Institute of Virology was one of the highest-profile events on their agenda because of the controversial theory it was the source of the pandemic.
Former US president Donald Trump and his secretary of state, Mike Pompeo, championed the theory last year, and it has come into sharper focus as Chinese secrecy and the inability to pinpoint a natural source raised suspicions.
Here are some key questions about the facility:
- What does the institute do? -
Its scientists conduct research on the world's most dangerous diseases and helped shed light on the Covid-19 pathogen in the early days of the outbreak in Wuhan.
Last February, they published work concluding that the new virus's genetic makeup was about 80 percent similar to the SARS coronavirus, and 96 percent identical at the whole-genome level to a coronavirus found in bats.
The lab's researchers had previously conducted extensive investigations into the links between bats and disease outbreaks in China.
Many scientists think the virus that causes Covid-19 originated in bats and may have jumped to people via another mammal, but there is no proof yet.
- Does it handle deadly viruses? -
Yes. The institute houses the largest virus bank in Asia, which preserves more than 1,500 strains.
The complex contains Asia's first maximum-security lab equipped to handle Class 4 pathogens (P4) such as Ebola.
The 300-million-yuan ($42 million) P4 lab opened in 2018. A P3 lab -- the biosafety level that includes coronaviruses -- has been in operation since 2012.
- What do we know about the virus origin? -
Many leading scientists declared early on that the pathogen appeared to be of natural origin, and there is little dispute that it first spread widely in late 2019 at a wet market in Wuhan where wildlife was sold as food.
But the trail ends there, and some unconfirmed clues -- aggressively seized on by China's government -- have suggested that its origins may even predate Wuhan.
A study by a group of Chinese scientists published in The Lancet in the pandemic's early days found that the first Covid-19 patient had no connection at all to the market, and neither did 13 of the first 41 confirmed cases.
- Why is a lab leak a candidate? -
Previous US diplomatic cables reported by the Washington Post had revealed concern in Washington about safety standards in the Wuhan facility.
Shi Zhengli, one of China's leading experts on bat coronaviruses and deputy director of the P4 lab, further raised eyebrows in a June 2020 interview with Scientific American magazine in which she said she was initially anxious over whether the virus had leaked from her lab.
Subsequent checks revealed that its gene sequence differed from viruses held at the lab, said Shi, adding, "I had not slept a wink for days."
Shi later said she would "bet her life" that there was no leak, according to Chinese state media.
The theory was pushed into the mainstream by Trump and Pompeo.
Pompeo insisted last year that there was "significant evidence" that the virus came from the lab, but he offered no proof and acknowledged that there was no certainty.
- Why does the lab-origin theory persist? -
With no progress in pinpointing the virus's source more than a year after the outbreak, the lab theory has gained a new lease of life.
In early January, a lengthy article in New York Magazine examining that possibility in detail refocused attention on it.
Other prominent global publications, including Le Monde and the Wall Street journal, as well as scientists at Harvard and Stanford, have published articles or reports saying the lab theory is one possibility.