On the same day, the WHO group, composed of 10 experts, observed for more than four hours a provincial disease control center that dealt with the virus spread in its initial stage.

Also read: WHO teams visits Wuhan food market in search of coronavirus clues

The center with a high level of research equipment has analyzed the genetic code of the virus, local media said.

On Sunday, meanwhile, the WHO team conducted a one-hour investigation into a market in the central Chinese city, where many people were confirmed to have contracted the virus in the early days of the outbreak in late 2019.

But it remains uncertain whether the probe would help identify the origins of the virus, as more than a year has passed since the first infection case was spotted in Wuhan, a business and transportation hub with a population of around 11 million.

The Wuhan market, at which wild animals such as bats and snakes were sold alongside seafood, has been closed since January 2020 and been apparently sanitized thoroughly by the Chinese authorities.

Also read: Relative of virus victim asks to meet WHO experts in Wuhan

The possible investigation into the research laboratory may become key to tracing the source of the virus that causes the COVID-19 disease, some medical experts say.

Last year, then U.S. President Donald Trump said he was confident the virus had originated from the infectious diseases lab and threatened to punish China for what he perceived as its lackluster response that allowed it to spread worldwide.

The WHO team had initially planned to visit China early last month, but its arrival was delayed after it took the experts longer to gain permission to enter the country.

After quarantining for two weeks, the WHO experts 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 said.

Although the WHO sent a small group of experts to China for a preliminary probe in July last year, they did not visit the market or the laboratory in the city at that time.

Also read: WHO team arrives in Wuhan to investigate pandemic origins

Since first being detected in Wuhan, the virus erupted into a pandemic that has infected over 100 million people and caused more than 2.2 million deaths across the globe, according to data compiled by Johns Hopkins University.