The coronavirus was already spreading widely throughout Wuhan, China, by December of 2019 and had already mutated into more than a dozen strains before the end of that year, World Health Organization (WHO) investigators said Monday.

CNN reported that a team of experts probing the virus’s origin in China said that they had found evidence indicating that the spread of COVID-19 in Wuhan had been far more widespread in late 2019 than Chinese authorities admitted at the time.

“The virus was circulating widely in Wuhan in December, which is a new finding,” Peter Ben Embarek, the WHO’s lead investigator on the mission, told CNN.

Ben Embarek added to CNN that the team had identified the first known case of COVID-19 in China, which he said had no link to the markets originally blamed for the virus’s spread.

“He has no link to the markets,” Ben Embarek told the news channel. “We also spoke to him. He has a very — in a way — dull and normal life, no hiking in the mountains type of things. He was an office worker in a private company.”

News of the virus’s more widespread transmission in Wuhan comes just a day after the Biden administration called on China to provide all data it has gathered related to COVID-19 and stressed the administration’s “deep concerns” regarding reports indicating that China was stymieing WHO investigators.

“We have deep concerns about the way in which the early findings of the COVID-19 investigation were communicated and questions about the process used to reach them,” White House national security adviser Jake Sullivan said. “It is imperative that this report be independent, with expert findings free from intervention or alteration by the Chinese government. To better understand this pandemic and prepare for the next one, China must make available its data from the earliest days of the outbreak.”

Officials on the WHO team have blamed China’s government for allegedly pressuring them to say that the coronavirus was imported to China from abroad, a counter to the Trump administration, which spent much of 2020 attempting to blame China for the virus’s spread, even often calling it the “China virus.”