Chinese authorities say they have recovered hundreds more dead pigs from Shanghai's main waterway, bringing the total to more than 6,600.
City workers continued to retrieve carcasses Wednesday from the Huangpu river, Shanghai's major source of drinking water.
Officials say the dead pigs have not contaminated the city's water supply, noting the river is no more polluted than normal.
But local authorities are puzzled about how the pigs ended up in the Huangpu river.
Authorities say ear tags from the some of the pigs suggest they floated downstream from the Jiaxing area in the neighboring province of Zhejiang, a major center for hog raising.
But Zhejiang animal husbandry and veterinary official Jiang Hao said that is not necessarily the case, since the ear tags only indicate where the pigs were born.
"We have a procedure for the labels on pigs' ears. After pigs are born, we tag the labels on their ears once they've been through the epidemic prevention process. After they have been given a label, they might leave here and move somewhere else as they move around the market."
It is not clear how the pigs died, though some have tested positive for porcine circovirus, a common swine disease that does not affect humans.
The Huangpu River supplies nearly a quarter of the water for Shanghai, China's financial hub and a city of 20-million people.
Water contamination, often caused by fertilizer runoff, chemical spills and untreated sewage, is a major problem in China. The government plans to spend $850 billion during the next decade to improve water supply systems.
Some information for this report was provided by AP, AFP and Reuters.