Pork buns and tap water may be off the menu in Shanghai, China’s biggest city with more than 23 million people, after thousands of dead pigs were found floating in the Huangpu River, which flows through the city, and in upstream tributaries. About 6,000 animals have been fished out so far in an operation that began last Friday, according to the Shanghai authorities, with more still surfacing, though at a slower pace.

The questions around the pig die-off — what caused it, why the animals were thrown into the river and by whom — are deeply disturbing Shanghai residents as well as others in China, and the Ministry of Agriculture has announced an investigation. City water authorities say the drinking water sourced in the Huangpu is safe, though one water sample showed traces of porcine circovirus, Xinhua, the state news agency reported, adding it can spread among pigs but not humans.

The surge in dumping of dead pigs — believed to be from swine farms in the upstream Jiaxing area of neighboring Zhejiang province — has followed police campaigns against the sale of pork products made from diseased pigs.

On Wednesday, a Zhejiang court sentenced 46 people to jail for producing unsafe pork from sick pigs that they had acquired and slaughtered between 2010 and 2012.

The official Xinhua News Agency said police in the city of Wenling had seized 6,218 kilograms of diseased pork.

In another operation last year, police in Jiaxing broke up a gang that acquired and slaughtered diseased pigs. Provincial authorities said police arrested 12 suspects and confiscated nearly 12 tons of tainted pork.