TAIPEI, Taiwan — A World Health Organization mission to discover the origins of the coronavirus got off to an inauspicious start on Thursday when two members of the team were barred from entering China.

The group of scientists arrived in Wuhan to begin the long-awaited investigation, according to the WHO, but two researchers were kept in Singapore, where they were transiting, and prevented from continuing to Wuhan after failing to clear health checks. China requires all airline passengers to present negative antibody tests before being able to travel to the country.

The pair had tested positive for coronavirus antibodies in their home countries before leaving. All members of the team, tested before leaving and once again in Singapore, tested negative for the virus.

The team’s partial arrival comes after Chinese authorities delayed the trip by more than a week, and after almost a year of wrangling and negotiation by the WHO to gain access to the central Chinese city where the coronavirus emerged.

The investigators — a range of virus, food safety and other experts — face a battle to find the source of the pandemic more than a year after the mysterious virus was first detected. They will also grapple with political hurdles and added scrutiny as critics accuse the U.N. health body of being beholden to China, whose state media has been promoting the narrative that the virus originated outside the country.

WHO researchers have said they are committed to following “all leads,” while Beijing has pledged willingness to cooperate, despite initial opposition to an international probe. Chinese officials said this week that the international team will work with Chinese researchers in the investigation, raising questions about the team’s freedom to pursue all lines of inquiry.

The mission faces the added complication of a spike in virus cases in China, where authorities on Thursday reported the country’s first coronavirus death in eight months. Health officials said a woman in Zengcheng village in Hebei province, the epicenter of a new outbreak near Beijing, died Wednesday from organ failure caused by the virus.

After experiencing a mild fever, coughing and breathing problems, the woman tested positive for the virus on Jan. 9 and was hospitalized. Hebei health officials did not give the woman’s age but have previously said all critical cases in the province were patients over age 60. Officials said she had underlying, severe post-cardiac injury syndrome.

China, which celebrated victory over the virus last year with businesses and schools reopening and awards handed out to virus experts and front-line medical workers, is now battling resurgent cases. The National Health Commission on Thursday reported 138 new infections, the largest daily increase since last March.

On Thursday, authorities were marshaling resources and manpower to Hebei province, where more than 20 million people in several cities within commuting distance of Beijing have been placed under lockdown. At least 3,000 quarantine units arrived in the provincial capital of Shijiazhuang, while workers were constructing field hospitals in Beijing.

According to official records, less than 5,000 people have died in China from the coronavirus — a total that critics say has probably been understated given the number of deaths in the early days of the outbreak before testing kits were widely available.

On social media, news of the country’s first covid-19 death since mid-2020 was one of the most discussed topics, with users asking for more details of the woman’s case. One commentator wrote: “Why does the news only say that there was one death in Hebei? How old is she? Why did they fail in saving her?” 

Lyric Li in Seoul and Alicia Chen in Taipei contributed to this report.