More than 100 people were evacuated from a 35-storey Hong Kong housing block on Tuesday after two residents in different apartments tested positive for the new coronavirus.
Residents were forced to leave in the early hours as health officials in masks and white overalls scrambled to work out whether the virus had spread through the complex of some 3,000 people.
Hong Kong is on high alert for any potential "super spreader" events, especially in the towering housing blocks that make the city one of the world's most densely populated places.
During the 2003 outbreak of Severe Acute Respiratory Syndrome (SARS), which killed 299 people in Hong Kong, 42 deaths came from just one housing block where about 300 people were infected.
In that outbreak, the virus was found to have spread through faulty drainage pipes.
Officials said Tuesday's relocation of residents in Tsing Yi district was a precautionary measure after a 62-year-old woman on the third floor was found to have the virus. She lived 10 floors below another man who is also a carrier.
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"We are not sure what was the exact route of transmission," Wong Ka-hing, from the Centre for Health Protection, told reporters.
"It could still be through the usual method of droplets or contact." Nonetheless the occupants of 35 flats connected to the same drainage system were moved out.
Health secretary Sophia Chan said four residents have since shown flu-like symptoms and have been taken to a hospital isolation ward. The others were taken to quarantine camps.
Residents on Tuesday morning found their neighbourhood filled with police and health officials.
"Of course I'm scared," a 59-year-old resident, who gave her surname as Chan, told AFP.
"I live with my son, daughter-in-law, grandchildren and my husband. We seldom go out already because we don't have enough masks. I don't allow my grandchildren to play in the hallway. Now we can't even stay at home."