For 27 days they searched for clues in Wuhan, visited hospitals, live animal markets and government laboratories, conducted interviews and pushed Chinese officials for data, but an international team of experts left the country far from understanding the origins of the coronavirus pandemic worldwide 2.8 million people killed.

The 124-page report of a joint World Health Organization-China investigation, due to be officially released on Tuesday and released to the media on Monday, contains a plethora of new details but no profound new evidence. And it does little to allay Western concerns about the role of the Chinese Communist Party, which is known to be resistant to outside control and has at times tried to prevent an investigation by the WHO. The report is also not clear whether China will allow outside experts to dig further.

“The investigation is in danger of getting nowhere and we may never find the true source of the virus,” said Yanzhong Huang, senior fellow on global health with the Council on Foreign Relations.

The report, the advance copy of which was obtained from the New York Times, said China still lacks the data or research to indicate how or when the virus began to spread. Some outside of the country skeptics say China may have more information than it admits.

The team of experts also dismissed the possibility that the virus accidentally emerged from a Chinese laboratory as “extremely unlikely,” although some scientists say this is an important question that needs to be investigated.

The Chinese government has tried to provide some level of access and cooperation, but has repeatedly tried to bend the investigation to its advantage. The report was co-authored by a WHO-selected team of 17 scientists from around the world and 17 Chinese scientists, many of whom hold official positions or work in government-run institutions, which has given Beijing great influence on its conclusions.

The Chinese scientists provided all of the research data used in the report, while the overseas scientists reviewed and interviewed Chinese researchers, doctors, and patients. It is not clear whether the team selected by WHO requested access to other data or permission to collect more data.

Jesse Bloom, an evolutionary biologist at the Fred Hutchinson Cancer Research Center in Seattle, said he was not convinced that a laboratory leak was extremely unlikely after seeing a copy of the report. He said he agreed that it was highly plausible that the virus would naturally have evolved to spread to humans, but he saw no reason in the report to rule out the possibility of a laboratory escape.

A member of the expert team, Peter Daszak, a British disease ecologist who heads the EcoHealth Alliance, a New York-based pandemic prevention group, backed down against criticism of the team’s work and collaboration in China. He said the laboratory leak hypothesis was “political from the start”. Dr. Daszak added that the WHO team was not constrained in its interviews with scientists who were on-site at the beginning of the pandemic.

He himself has been accused of having a conflict of interest for doing a past research on coronavirus with the Wuhan Institute of Virology, what a disease ecologist should do.

Updated

March 29, 2021, 10:36 p.m. ET

“We were in the right place because we knew there was a risk of the virus occurring,” said Dr. Daszak. “We worked with that same viral group there and it happened.”

The prevailing theory remains that the virus came from bats, jumped to another animal, and then mutated in a way that allowed it to be transmitted to humans and from person to person. However, the process of tracing the origins of a virus is notoriously tedious.

To answer many of the remaining questions, the report recommends further retrospective studies of infections in humans, including the earliest cases, as well as further virus testing in farm animals and wildlife in China and Southeast Asia. It also calls for more detailed tracking of routes from farms to markets in Wuhan, which would require extensive interviews and blood tests for farmers, vendors and other workers.

It is unclear how much China will cooperate, however, and the country’s secretive and defensive behavior has helped fuel theories that were somehow responsible for starting the pandemic. Local officials in Wuhan first tried to hide the outbreak; Beijing has since expelled many Western journalists and put forward evidence-free theories about the virus originating elsewhere – although the earliest known cases were all in China and experts believe it almost certainly showed up there first.

“We have real concerns about the methodology and process that went into this report, including the fact that the Beijing government appears to have helped write it,” Foreign Secretary Antony J. Blinken said in a CNN interview that aired on Sunday.

China’s increasingly keen ties with the United States and other countries have also made investigation difficult. The Biden government has repeatedly criticized China’s lack of transparency, including its refusal to provide raw data on early Covid-19 cases to investigators during their visit to Wuhan. Chinese officials have resisted suggesting that the United States should welcome WHO to investigate the unsubstantiated theory that the virus may have originated in a US Army laboratory.

“We will never accept the baseless allegations and wanton denigration of the United States regarding the epidemic,” said Zhao Lijian, a spokesman for the Chinese Foreign Ministry, at a regular press conference in Beijing on Monday.

In bombastic news articles, Chinese propagandists have hailed the investigation as a sign of China’s openness to the world and as a justification for the government’s handling of the epidemic.

WHO has come under pressure to request more data and research from the Chinese government. However, the global health authority is inherently obliged to its member countries, which have not given the WHO team extensive powers to conduct forensic investigations into laboratory accidents in China, for example.

While much of the report was in-depth about molecular studies, virus development, and possible animal hosts, the section on the possibility of a laboratory leak was sketchy at best. While the animal origin of the virus is largely undisputed, some scientists claim that the virus could be collected and present in the laboratory of the Wuhan Institute of Virology, although Chinese scientists do not claim it is.

China’s lack of transparency and other concerns prompted a small group of non-WHO scientists to call for a new investigation into the origin of the pandemic this month. They said such an investigation should consider the possibility that the virus escaped from or infected someone in a laboratory in Wuhan.

The laboratory leak theory was promoted by a number of Trump administration officials, including Dr. Robert Redfield, the former director of the Centers for Disease Control and Prevention, endorsed it in comments on CNN last week. He did not provide any evidence and insisted that it was his opinion.

Matt Apuzzo and Apoorva Mandavilli contributed to the coverage. Albee Zhang contributed to the research.