World Health Organization’s mission to study the origin of the coronavirus arrived in Wuhan, China, on Thursday, but two researchers are stuck in Singapore awaiting further test results.

“All team members had multiple negative PCR and antibody tests for COVID-19 in their home countries before traveling,” WHO tweeted. “They were tested again in Singapore and were all negative for PCR. But two members tested positive for IgM antibodies. They are being retested for both IgM and IgG antibodies.”

Chinese Foreign Ministry spokesman Zhao Lijian said that the country “will strictly follow the relevant epidemic prevention regulations and requirements, and provide corresponding support and facilities for WHO experts who come to China to carry out international cooperation on tracing the origin of the virus.”