Chinese officials on Thursday blocked two members of the World Health Organization (W.H.O.) team investigating the origins of the Chinese coronavirus from entering China.

The two team members supposedly tested positive for Chinese coronavirus antibodies when their blood was tested. The remaining 13 members of the team said they would continue to Wuhan, ground zero for the pandemic, and begin working while they observed a two-week mandatory quarantine.

The two banned scientists were prevented from boarding a flight to China from Singapore. As the South China Morning Post (SCMP) pointed out on Thursday, W.H.O. does not officially recognize the testing standard invoked by Chinese officials to ban them:

The two scientists were negative for the standard nucleic acid test used to diagnose active infection, but tested positive for IgM antibodies, which may point to recent infection.

China requires passengers flying from Singapore to take a nucleic acid test and an IgM antibody test up to two days before boarding China-bound flights. WHO guidance, however, says antibody tests should not be used to diagnose acute Covid-19 infection, as antibodies develop a few weeks after infection.

The WHO said the two scientists were still in Singapore to be retested for IgM antibodies and another type, IgG antibodies. Those are thought to be longer lasting than IgM and suggest a person was previously infected.

“All team members had multiple negative PCR and antibody tests for Covid-19 in their home countries prior to travelling,” W.H.O. pointed out, while medical experts quoted by the SCMP said the testing method employed by China has a strong tendency to produce false-positive results.

“The 10 international scientists taking part in the mission include public health experts, animal health specialists and virus hunters from Japan, Qatar, Germany, Vietnam, Russia, Australia, Denmark, the Netherlands, Britain and the United States. In addition, the 15-person delegation meant to fly to Wuhan from Singapore includes experts from the WHO and the World Organization for Animal Health (OIE),” the SCMP reported. It did not specify the nationality or background of the two members barred from entry by China.

China blocked two members of the W.H.O. team last week due to vague “visa clearance” issues. Before that, China put the entire W.H.O. mission on hold because it said their permission to enter China had not been “finalized,” prompting a rare expression of disappointment from W.H.O. Director-General Tedros Adhanom Ghebreyesus. Tedros was obliged to contact senior Chinese officials and “fully impress upon them the absolute critical nature” of the coronavirus investigation.

“The early snag adds a further complication to a mission already fraught with political sensitivities, as Beijing has waged a broad campaign to cast doubt on the origins of a pathogen that was first discovered in central China. The WHO and Beijing have spent months negotiating the terms of the mission, which has been delayed several times, and the United Nations agency’s researchers say they don’t have a clear understanding of the progress that Chinese scientists have made in what is meant to be a collaborative effort,” the Wall Street Journal noted.