In March, more than 16,000 dead pigs showed up in a stretch of the Huangpu River — a main source of the city's drinking water.

Local officials insisted both the water and the city's pork supply were safe, but they never explained exactly how the pigs died or how they ended up in the river. One working theory was that farmers upstream in neighboring Zhejiang province dumped the dead pigs after officials there cracked down on the practice of selling diseased pork to local markets.