Thousands of pig carcasses were seen floating in a key water source in Shanghai, sparking public outcry over food safety concerns and China’s worsening environmental pollution.

Nearly 3,000 dead pigs were found in East China’s Huangpu river and nearby waters in recent days. By Monday afternoon, Chinese authorities say they had fished 3,300 carcasses from the river.

Little was known on what killed the pigs and how so many of the dead animals ended up in the river, which supplies water to Shanghai’s 23 million residents.

The Ministry of Agriculture launched an urgent investigation into the deaths of the pigs and any potential impact to the water source, China Daily reports.

Initial investigation have found no evidence of widespread animal disease in the immediate area, according to Chen Xiaohua, vice-minister of agriculture, although tests have revealed some pigs may have died from porcine circovirus (PCV). The virus cannot affect humans, the Shanghai Animal Disease Control and Prevention Center said.

Shanghai authorities said that the animals came from Jiaxing in Zhejiang province, citing the animals’ ear tags, where hundreds of pigs reportedly died in the first two months of the year.

A spokesman for the Jiaxing government said the city has procedures for collecting dead livestock but said he can’t rule out that some farmers simply dumped the pigs in the river.

Shanghai residents voiced outrage over the incident, claiming the government was slow to respond to the crisis which broke out on Friday.

The Shanghai government said they were continuing recovery of pig carcasses from the river and was closely monitoring the water quality, Reuters reports.