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BEIJING (NewsNation Now) — The World Health Organization’s report on the origins of the coronavirus following a mission to Wuhan, China, was released Tuesday, but Director Dr. Tedros Adhanom Ghebreyesus says further study is required.

The review, which was conducted for a WHO team on international experts in Wuhan, China, between Jan. 14 and Feb. 10, is considered a first step in what will likely become a years-long investigation into the virus’ origins.

The report points to transmission from bats to another animal and subsequently to humans as the most likely way the pandemic began.

Although many early cases of COVID-19 were connected to a seafood market in Wuhan where animal products were sold, the report found that a similar number of cases were associated with other markets or no markets at all. Extensive testing at the Huanan seafood market found no evidence of SARS-CoV-2 in those animals. However, viruses from some of the Huanan market cases were identical, the report detailed, suggesting the market contributed to spreading the virus — an early super spreader event — even if it did not originate there.

“No firm conclusion therefore about the role of the Huanan market in the origin of the outbreak, or how the infection was introduced into the market, can currently be drawn,” according to the report.

Peter Ben Embarek, the WHO expert who led the mission to Wuhan, China, said it was “perfectly possible” COVID-19 cases were circulating in November or October 2019 around Wuhan, potentially leading to the disease spreading abroad earlier than documented so far.

He said the team felt political pressure, including from outside China, but that he never was pressed to remove anything from its final report.

Tedros said on Tuesday that data was withheld from WHO investigators. He made the comments to the agency’s member states, in a separate press briefing.

China refused to give raw data on early COVID-19 cases to the WHO-led team, one of the team’s investigators has already said, potentially complicating efforts to understand how the global pandemic began.

“In my discussions with the team, they expressed the difficulties they encountered in accessing raw data,” Tedros said. “I expect future collaborative studies to include more timely and comprehensive data sharing.”

The conclusions that the virus origins remains incomplete likely means that tensions over how the pandemic started – and whether China has helped or hinder efforts to find out, as the United States has alleged – will continue.

Although the team concluded a leak from a Wuhan-area laboratory was the least likely hypothesis for the virus that causes COVID-19, Tedros said, the matter requires further investigation, potentially with additional missions back to China.

More than 20 heads of government and global agencies called in a commentary published Tuesday for an international treaty for pandemic preparedness that they say will protect future generations.