LONDON — China is preparing to test tens of thousands of blood bank samples from the city of Wuhan as part of a probe into the origins of Covid-19, CNN reported on Wednesday quoting a Chinese official. The move comes amid increasing calls for transparency over the emergence of the virus.

The store of up to 200,000 samples, including those from the closing months of 2019 were pinpointed in February this year by the World Health Organization's panel of investigators as a possible source of key information that could help determine when and where the virus first crossed into humans.

The samples are kept in the Wuhan Blood Center, and are thought to span 2019, providing real-time tissue samples from a wide swathe of the population in the Chinese city where SARS-CoV-2 is thought to have first infected humans.

The blood bank samples have been retained for two years, Chinese officials have said, in case they are needed as evidence in any lawsuits related to the blood donations they are from.

That two-year waiting period will soon expire for the key months of October and November 2019, when most experts think the virus could first have infected humans. An official from China's National Health Commission, told CNN preparation for testing is currently underway, and confirmed testing would happen once the two-year limit was reached.

"This provides the closest in the world we've seen of real time samples to help us understand the timing of the outbreak event," said Yanzhong Huang, senior fellow for global health at the Council on Foreign Relations.

They "absolutely will contain vital clues," said Maureen Miller, associate professor of epidemiology at Columbia University. She urged China to permit foreign experts to observe the process. "No one will believe any results that China reports unless there are qualified observers at the very least," she said.

The head of the Chinese team working on the WHO investigation, Liang Wannian, first said in a July news conference that China would test the samples, adding once the Chinese experts "have the results, they will deliver them to both the Chinese and foreign expert teams."

Liang said the samples came from the opening tube of a donor blood pouch, sealed shut and then stored, and Chinese experts had "made several assessments and evaluations on the testing methods and action plan, which will be implemented after the expiry" of the two-year limit.

The samples, if stored correctly, could contain crucial signs of the first antibodies made by humans against the disease, experts said.

Liang said in July that while the first reported case was in Wuhan on December 8, "our research and the previous related research papers of Chinese scientists fully suggest ... December 8 is probably not the primary case. There might be other cases that occurred before."

Dr. William Schaffner, from the Vanderbilt University Department of Medicine's infectious disease division, said the samples presented a "fascinating opportunity. You would like to go back to find out exactly during which months this this virus started to leave fingerprints in the human population in China."

The samples might even indicate who was first infected, where, and their age and occupation, Miller added.

"It is common practice to de-identify the samples," she said. “So you could strip it down to basic demographics, age, gender neighborhood where they lived. All of those data will be available."

Schaffner suggested the samples could be brought to Geneva, or another neutral destination, to permit WHO experts to take part in the testing.

He said two possible issues with the samples could be "the integrity of the blood samples -- ensuring they had not been recently created," but also how representative of the population as a whole the blood donors were. Miller said many samples would most likely have been taken from healthy individuals "so they'll represent asymptomatic cases. And as we've learned over the course of the pandemic, asymptomatic cases fuel the pandemic."

Huang said it was unclear to "what extent the outside world would trust the findings as credible or convincing," and the testing marked an opportunity for China to "tell the world that they are serious about depoliticizing the origins probe."

The Biden administration conducted a 90-day review of the intelligence over how the virus originated, yet an unclassified report had officials still considering both natural transmission from animal to humans and a lab leak as plausible theories, yet unable to determine which was the more likely.

President Joe Biden, on receiving a classified version of the report, said: "Critical information about the origins of this pandemic exists in the People's Republic of China, yet from the beginning, government officials in China have worked to prevent international investigators and members of the global public health community from accessing it."

China has insisted it has been transparent and helpful to the WHO probe, and in its most recent statement about the theory the virus leaked from a laboratory, pointed toward unproven claims about Fort Detrick, US laboratory in Maryland, and the need to examine its recent past.