In China’s largest single-day increase in coronavirus infections since April, 137 people in the northwestern city of Kashgar tested positive Sunday.

All are coworkers of the parents of a 17-year-old girl who tested positive on Saturday during a routine check at her place of work, Kashgar health authorities said during a press conference Sunday. All 138 people are asymptomatic.

Kashgar, located in the Xinjiang Uyghur Autonomous Region, has activated its highest-level emergency response. Local authorities plan to test all of the city’s 620,000 residents within two days. As of Sunday morning, medical staff had taken samples from over 300,000 people.

The girl’s parents and brother tested negative, as did 831 people at her place of work, described during the press conference as a garment factory where she lived most of the time and left for home about once every two weeks.

The source of the outbreak is unclear. According to state-run newspaper People’s Daily, the 17-year-old had not been in close contact with people with confirmed or suspected infections, people with fevers, or wild animals during the two weeks before Saturday.

Xinjiang last saw a resurgence of COVID-19 cases in July, one of the handful of flare-ups in China since its initial outbreak subsided. Then, over 890 total cases were reported in the region.

Editor: Kevin Schoenmakers.

(Header image: People line up for nucleic acid tests in Kashgar, Xinjiang Uyghur Autonomous Region, Oct. 24, 2020. People Visual)