The World Health Organization’s mission to trace the origins of COVID-19 got off to a rocky start on Thursday, with two members of the 15-person delegation denied entry to China after failing to clear health-screening procedures.

The two experts were stopped from boarding a plane to the city of Wuhan after testing positive for COVID-19 antibodies in blood-based serology tests during transit in Singapore, even though they tested negative for COVID-19 in swab tests, Dow Jones reported, citing unidentified people familiar with the matter. The WHO later confirmed that two scientists were stuck in the city-state as video showed the others arriving in Wuhan around noon.

Chinese Foreign Ministry spokesman Zhao Lijian sidestepped questions about the two scientists who were denied entry and how long the group members will be quarantined, but said their visas will be valid “long enough for them to conduct their work in China.” The Communist Party-backed Global Times reported earlier the quarantine period would last 14 days.

“The international experts’ team will undergo quarantine in Wuhan, during which time they will conduct video conference with Chinese experts,” Zhao told a regular briefing in Beijing on Thursday afternoon.

Peter Daszak, the only American on the mission, arrived in China after testing negative, according to a spokesman for the EcoHealth Alliance, the research organization that he runs.

The experts will work with Chinese scientists to determine the source of the pandemic, as they still store the strains for analysis and study purposes, Ihor Perehinets, a technical adviser at the WHO’s European office, said at a media briefing. His comments were in response to a question on how the team would be able to identify the source after a year has already passed since the start of the outbreak.

China in November put in place a requirement for negative results of both a nucleic acid test and antibody test for all inbound travelers as it tightened safeguards to ensure people arriving from abroad don’t bring in the coronavirus.

While nucleic acid tests can detect the virus on a sample taken from a person’s respiratory track, there’s a chance the result could give a false negative if the viral load where the swab was taken is below the lowest threshold for detection. An antibody test could indicate that a person is in the acute phase of infection when testing positive for the IgM antibody, which is the first wave of antibodies generated to fight infection.

Earlier this month, WHO expressed disappointment with China for delaying the travel of experts to the country to investigate the origin of the virus, in a rare instance of public criticism from the international organization.