The city government of Huzhou, Zhejiang province, confirmed on Sunday that investigators have dug out the remains of hundreds, and possibly thousands, of rotting carcasses of diseased pigs from three burial sites.

The public security bureau of Huzhou detained five people suspected of the illegal burial of the sick pigs in 2013, and will continue to investigate, according to a statement posted on the city's micro blog account.

The city was told on Aug 30 by environmental inspectors from the central government that some dead pigs had been illegally buried in Santianmen village. The inspectors had been evaluating Zhejiang province since Aug 11 and received reports about the pigs from members of the public.

The city's environment inspectors dug up the decomposed carcasses on Aug 30, and then continued digging in two other places identified by the suspects and by others who first tipped off the authorities, the statement said.

As of Friday, investigators had dug in three places and "finished the sorting process", which yielded "2.24 million metric tons" of material including decomposed pig carcasses and sludge, it said.

"All of them have been removed and burned to protect the biological safety of the environment," it added.

In addition, the city's environmental bureau tested the quality of soil and water both at the surface and underground, and health authorities took steps to decontaminate the site.

An initial investigation by the city's public security bureau found that Huzhou Industrial and Medical Waste Treatment Co had dumped and buried the pigs, which died of disease in 2013 and should have been burned to protect the environment.

Investigators said Shi Zheng, who was manager of the company at the time, ordered the workers to bury the pigs. Shi is currently serving a prison sentence for other crimes. Five other suspects in this case have been detained by the police.

China's eastern region is a major pig breeding area. Zhejiang and Jiangsu provinces have issued rules to govern the disposal of dead ones.

The case that came to light on Sunday is not the first case involving diseased pigs. In March 2013, thousands of dead pigs were found floating in a section of Shanghai's Huangpu River. City authorities dragged 5,916 carcasses from the river and the municipality took emergency measure to protect water quality, according to a report by Caixin on Sunday.