Dr. Sachs, chairman of a Covid-19 commission affiliated with the Lancet scientific journals, said he closed the task force because he was concerned about its links to EcoHealth Alliance. The New York-based nonprofit has been under scrutiny from some scientists, members of Congress and other officials since 2020 for using U.S. funds for studies on bat coronaviruses with the Wuhan Institute of Virology, a research facility in the Chinese city where the first Covid-19 outbreak occurred.

EcoHealth Alliance’s president, Peter Daszak, led the task force until recusing himself from that role in June. Some other members of the task force have collaborated with Dr. Daszak or EcoHealth Alliance on projects.

“I just didn’t want a task force that was so clearly involved with one of the main issues of this whole search for the origins, which was EcoHealth Alliance,” said Dr. Sachs, who is also director of the Center for Sustainable Development at Columbia University.

One task-force member said the disbanded group doesn’t have conflicts of interest that interfere with its ability to gather and assess data on how the virus got into human populations.

Disbanding the task force is the latest development in the global scientific quest to find the origin of the virus. The search has fueled geopolitical tensions and debates over whether the virus began spreading after spilling into the human population naturally, from an animal, or from a laboratory or other scientific research-related accident. Calls from scientists and health leaders for an investigation into the possibility of a lab accident have gained traction in recent months.

Dr. Sachs said the Lancet Covid-19 Commission would continue studying the origins for a report to be published in mid-2022 but broaden its scope to include input from other experts on biosafety concerns including government oversight and transparency regarding risky laboratory research. More labs have the technology to re-create or build new viruses, he said, yet regulation and standards on how to conduct these experiments safely aren’t keeping up.

“A lot is going on around the world that is not properly scrutinized or explained to the public,” he said. He said he isn’t backing one hypothesis about the origins of Covid-19 over another. The task force had been pursuing leads on both the natural and laboratory-leak hypotheses.

Members of the task force that Dr. Sachs disbanded in mid-September are discussing ways to continue the investigations they had under way. “We are going to carry on with this important work,” said Stanley Perlman, a professor of microbiology and immunology at the University of Iowa and a task-force member.

Dr. Sachs appointed Dr. Daszak to lead the task force in 2020. The 12-member group is made up of experts in emerging viruses and animal health. Included are Malik Peiris, a Hong Kong-based virologist who played a critical role in identifying the coronavirus that caused the SARS outbreak in 2003; Carlos das Neves, director for research and internationalization at the Norwegian Veterinary Institute; and Danielle Anderson, an Australian virologist at the University of Melbourne who conducted research at the Wuhan institute in late 2019.

An expert on searching for emerging viruses in animals that could threaten humans, Dr. Daszak has been a vocal opponent of the hypothesis that the virus might have spread from a laboratory accident. He was a member of a World Health Organization-led team that visited Wuhan earlier this year and concluded that a laboratory leak was extremely unlikely.

Five task-force members joined Dr. Daszak in signing letters in the Lancet in February 2020 denouncing what they called conspiracy theories that the new coronavirus had been bioengineered and in July 2021 saying more evidence supported a natural origin of the virus. Dr. Perlman, a coronavirus researcher for four decades, signed the letters but said he hasn’t conducted research with scientists at EcoHealth Alliance or the Wuhan institute.

The WHO-led probe into the origins has been largely stalled for months. A report by U.S. intelligence agencies, delivered to President Biden in August, didn’t reach any definitive conclusion, in part because of a lack of data from China.

New research is providing potential clues about the virus’s origin. Researchers at the Pasteur Institute, a French nonprofit foundation, recently reported finding three coronaviruses in bats in caves in northern Laos near the border with China that closely resemble the pandemic virus and can infect human cells. The findings show that viruses similar to SARS-CoV-2, the pandemic virus, are circulating in nature and could infect people who come into contact with these bats, the researchers said.

A new national Covid-19 commission plans to join the search for the pandemic’s origin in the next few months, said Philip Zelikow, director of the commission’s planning group and a professor of history at the University of Virginia. The new commission, supported by several foundations, is forming a task force that will include experts in emerging disease epidemics and scientists with experience in and concerns about high-risk laboratory research, said Mr. Zelikow, who was executive director of the 9/11 Commission.

“We are very interested in the origins issue,” he said. “We all think there is a chance of getting possibly some cooperation about this if this is seen as a nongovernmental effort.”