LONDON (Reuters) – The origins of a new cluster of coronavirus infections in Beijing are not certain, World Health Organization officials said on Monday, describing as a “hypothesis” the claim that it might have been caused by imports or packaging of salmon.

Several districts of the Chinese capital put up security checkpoints, closed schools and ordered people to be tested for the coronavirus on Monday after an unexpected spike in cases linked to the biggest wholesale food market in Asia.

State-run newspapers reported that the virus was discovered on chopping boards used for imported salmon at Beijing’s Xinfadi market amid worries about a second wave of the pandemic in China.

Mike Ryan, head of the WHO’s emergencies program, said in a briefing that he would be “reticent” to say that packaging needs to be tested for the virus as a result of the new infections.