Of 102 confirmed cases in the city, at least 33 patients worked or shopped at a department store in the Baodi district, or had close contact with employees or customers, according to the Tianjin health authorities. Many of them had no history of travel to Wuhan, the city where the outbreak emerged.

Officials estimated that 11,700 customers had visited the shopping complex during a period in late January. The authorities said that those customers would be quarantined, and that the store itself, which they did not identify, had been sealed and disinfected.

It was not immediately clear how the authorities had tracked the shoppers, but health officials in the city have put out alerts on social media and on state news outlets urging residents to contact the government if they visited the store recently. News reports also said residents had been asked at various checkpoints in the city if they had been there.