Police raided an underground workshop Monday night that was producing oil from pork of questionable quality, Qingpu district police told the Global Times Tuesday.

A group of food safety activists reported the unregistered workshop to police Monday. Group members posted photographs online showing pieces of pork stacked in filthy containers or on the floor around the workshop. Some of the meat had already started to go bad. "The workshop ran from 10 pm to about 5 am each day," said Wang Gang, one of the activists. "There were four mills. Two produced cooking oil and the other two produced pork jerky."

Police are still investigating whether the workshop was producing the oil for consumption or industrial use, said a press officer surnamed Shi from the Qingpu District Public Security Bureau.

The workshop was hidden in the woods behind a restaurant, Wang said. A friend who lives nearby told him that it had been operating for at least two years.

Wang said his friend had hired a private investigator to look into the workshop for three months. "There were trucks delivering pig carcasses to the workshop every day," Wang said. "It also bought recycled oil from restaurants."

The photographs also showed workers boiling the meat in dirty cooking pots.

Workers packaged the jerky on the workshop's floor, but managed to remove it before police arrived, said an activist surnamed Ni, who was present at the scene.

Police have arrested several of the workshop's workers.