The administration of former US President Donald Trump advanced a theory the Wuhan Institute of Virology might be the origin of the pandemic-causing virus, claiming some researchers there developed symptoms consistent with COVID-19 in the fall of 2019. China rejects the allegation.

All eyes are on whether the probe by the WHO group will help identify the origins of the virus, with more than a year having passed since the first infection was identified in Wuhan, a central China business and transportation hub with a population of some 11 million people.

On Sunday, the 10-person WHO team also carried out a one-hour investigation into a Wuhan market, at which many people were confirmed to have contracted the virus in the early days of the outbreak in late 2019.

The market, where wild animals such as bats and snakes had been sold alongside seafood, has been closed since January 2020. It has since been sanitized thoroughly by Chinese authorities.

On Tuesday, the WHO team visited a provincial animal health facility in the city wearing protective suits, as animals are suspected of having initially transmitted the virus to humans.

The WHO has been trying to clarify the process of how bats, believed to be the main natural host of the virus, passed it to other animals, sources close to the matter said.

After quarantining for two weeks, the WHO team began a full-fledged probe in late January. They are likely to complete the investigation in Wuhan by the Feb. 11 start of the weeklong Lunar New Year holidays, the sources added.

The WHO sent a small group to China for a preliminary probe in July, but they did not visit the market or the laboratory.

Since first being detected in Wuhan, the virus erupted into a pandemic that has infected over 103 million people and caused more than 2.2 million deaths across the globe, according to data compiled by Johns Hopkins University.