BEIJING — More than 2,800 dead pigs have been found in a major river that flows through Shanghai, igniting fears among city residents of contaminated tap water, according to state news-media reports Monday.

Officials were trying to determine who had dumped the carcasses into the river, the Huangpu, which slices through the heart of Shanghai.

Some reports blamed farmers.

Officials were seeking to track the source of the pigs from marks on their ears, and a preliminary inquiry found that the dumping occurred in Zhejiang province, which is south of Shanghai and upstream on the Huangpu.

Photographs of the carcasses floating in the river were circulating widely on the Internet.

One photograph on the website of Global Times showed sanitation workers in orange vests and blue uniforms lifting carcasses from Hengliaojing Creek with long wooden poles.

An accompanying report, citing a Shanghai news website, said the first carcasses were discovered Thursday near a water-treatment plant in an area that is a protected water resource.

Their numbers increased quickly over the weekend, and the tally is expected to grow further as search barges return to Shanghai.

Shanghai Waterworks, which manages the city’s tap water, said Sunday night that the water still met drinking standards, according to Xinhua, the state news agency. Shanghai officials said the group was checking the water hourly.

“So far, water quality has not been affected, but we have to remove the pigs as quickly as possible and can’t let their bodies rot in the water,” Xu Rong, the director of Shanghai’s Songjiang District Environmental Protection Bureau, told Global Times, a state-run English-language newspaper.

Xu said samples of the pigs had been sent to an agricultural commission to determine what killed them. Officials will have answers within three days, he said.

A sample of the river water tested positive for porcine circovirus, which officials said does not spread to humans, Xinhua reported. One strain of the virus can cause pigs to waste.

Many Chinese are expressing growing concern over air, soil and water pollution.

In recent weeks, several official news organizations have run articles and editorials casting a spotlight on pollution of some of China’s major waterways.

In one prominent case, a 39-ton chemical spill Dec. 31 from a fertilizer factory in Shanxi province affected two other provinces downstream.

Local officials delayed reporting the chemical spill for five days.

A statement issued Monday by the Shanghai government and posted on its website said there were piglets in the river as well as large adult swine.

Residents in the Songjiang district, the area southwest of downtown Shanghai where most of the pigs were discovered, said this was not the first time they had seen dead pigs in the Huangpu.

But this time, the number was higher than in the past, according to the city government’s statement.

Songjiang district officials said they were gathering all the dead pigs in one place to safely dispose of them, Xinhua reported.