TAIPEI, Taiwan — After a 12-day visit, a World Health Organization mission to Wuhan appeared no closer Tuesday to solving the mystery of the pandemic's origins, reiterating that the coronavirus likely spread to humans from an animal and casting doubt on theories it leaked from a lab.

The group's findings — more than a year after the initial outbreak and after months of wrangling between China and the U.N. health agency — could be a small step toward understanding the roots of a global crisis.

The update is unlikely to satisfy U.S. officials and others around the world calling for greater transparency from China — and is unlikely to silence questions about whether the Geneva-based WHO is equipped to investigate at all.

At a news conference, the team of Chinese and international researchers said they found that the virus was spreading in Wuhan during December 2019 both inside and outside the Huanan Seafood Market. That suggested the market was not necessarily the original source of the outbreak, the scientists said.

The team also left open the possibility that the virus may have been transmitted to humans through frozen food — a once-fringe theory that Chinese officials have been touting as part of broader push to claim that covid-19, the disease caused by the coronavirus, did not come from China.

Notably, the WHO team dismissed as “extremely unlikely” another theory that the virus leaked from laboratories at the local Wuhan Institute of Virology (WIV). Peter Ben Embarek, the Danish food safety expert leading the WHO team, said his group was satisfied with answers about safety at the WIV and will not recommend further investigation into the possible links to the lab.

“Just saying that they have really good safety protocols is not an answer in my view,” said Marc Lipsitch, a professor of epidemiology at Harvard’s T.H. Chan School of Public Health, who was not among the scientists on the trip. “That alone does not put my mind at rest.”

It was never likely that the team would reach a definitive conclusion after two weeks in quarantine and less than two weeks of on-site investigation. But the dismissal of the lab theory, in particular, is likely to draw scrutiny.

Most researchers believe the virus passed through an intermediary animal host — such as pangolins — and evolved into a form that is easily transmissible among humans.

A smaller circle of experts says the possibility cannot be ruled out that the virus slipped out of the WIV, an institution that conducts work on coronaviruses sampled from bats. 

The mission was composed of Chinese and international researchers. The lead Chinese scientist, Liang Wannian, told reporters that none of the labs in Wuhan had worked with the SARS-Cov-2 strain, as the novel coronavirus is officially called, but on the virus’s distant relatives. 

Instead, Liang pointed to the possibility that the virus jumped across species in nature through intermediary hosts such as pangolins, cats or minks. Ben Embarek agreed that it was “most likely” the virus evolved in nature and spread to humans through an intermediary host.

Ben Embarek told reporters that the judgment was based on “long, frank, open discussions with researchers and management” at institutions including the WIV. The institute provided “detailed descriptions of the center’s research both present and past on all projects involving bats and coronaviruses and more advanced projects,” he said.

He added that he questioned WIV officials extensively about what they thought of the lab leak hypothesis. “They’re the best ones to dismiss the claims and provide answers to all the questions,” the WHO team leader said.

But that line of reasoning drew skepticism from outside experts.

“If the only information you're allowing to be weighed is provided by the very people who have everything to lose by revealing such evidence, that just doesn't come close to passing the sniff test,” said David A. Relman, a microbiologist at Stanford University.

Relman suggested that the WHO team should have sought complete, detailed records from the laboratories about their experiments and the raw genomic sequence data of their research going back a decade.

Raina MacIntyre, professor of biosecurity at the University of New South Wales in Australia, was also surprised to see the idea of a lab accident ruled out so quickly.

Without exploring all leads, she suggested, “we may never know the origins of this virus.”

The search for answers about the source of the pandemic had been fraught from the start.

It is clear now that China withheld critical information about the outbreak in Wuhan in the early days. The WHO, for its part, has faced criticism for failing to call out Beijing.

In February 2020, WHO researchers traveled to China to study the novel coronavirus. The team’s enthusiastic praise for Chinese officials led many to wonder whether the agency was up to the task of holding the country to account.

As the crisis in the U.S. worsened, the Trump administration seized on the critique and used it to shift blame, with top U.S. officials floating unproven claims about the origin of the virus — and Chinese officials firing back with unproven claims of their own.

In the months since, the U.N. health agency has been caught in the middle. China, however, is already trying to move on.

On Tuesday, Liang and an official from China’s National Health Commission declared the China leg of the WHO probe complete and called for its scope to be expanded globally to answer the origin question.

In recent days, Chinese state media have preempted Tuesday’s news conference with reports declaring Wuhan has been “cleared of guilt” as the suspected origin of the pandemic, with some echoing the Chinese Foreign Ministry in calling for an investigation into U.S. labs.

A key question going forward will be whether the WHO team is allowed to return to China. In a pre-trip interview with The Washington Post, Ben Embarek said he believed the search would require trips to different places in China.

It remains to be seen, however, whether China will grant the WHO access, said Yanzhong Huang, a senior fellow for global health at the Council on Foreign Relations.

“There will be more pressure to do more work in China,” Huang said. “I hope this going to be the starting point of a more in-depth, more comprehensive investigation.”

Rauhala reported from Washington.