Police in rural southwestern China have saved six men with learning difficulties from a brick factory where they were forced to do hard labor without pay, Sixth Tone’s sister publication The Paper reported Sunday.

The Sheng Heng brick kiln is located in Yangchang Township in hilly Yunnan province, home to many such factories. According to police, the mentally disabled men were frequently beaten and scolded, and were restricted in their freedoms. Physical examinations after their release revealed that two of the men had burn wounds on their hands but were otherwise healthy.

The person in charge of the kiln, Qiu Xuerui, was detained and has since been released pending an investigation. The local government has now closed the kiln.

Authorities first reported the case in August but did not provide any specific dates. Police in Yangchang declined to comment when contacted by Sixth Tone on Sunday. An unnamed local government employee told The Paper that this was an isolated case, although villagers living nearby said mentally disabled kiln workers were common in the area.

Of the six people rescued by police, five didn’t know the names of their hometowns. They are being housed in a rescue center. The authorities have returned the sixth person, Bai Wanjia, back to his home some 500 kilometers away, also in Yunnan province.

Bai’s father told The Paper that his son has been working constantly since his return, saying, “If I don’t work, the boss will hit me.”

Human traffickers often prey on the mentally disabled and sell them into slavery or marriage. A 2011 China Daily report found that one person can earn a trafficker up to 500 yuan ($75). Bai was never paid, but he told The Paper the boss of the brick kiln had promised him a salary of 0.02 yuan per brick.

Cases of brick kiln workers being freed from abominable working conditions frequently make the news in China.

People’s Daily in 2013 reported on the case of 10 developmentally disabled people who had been tricked into slavery in a brick kiln. The workers came from areas near Yangchang Township. A year earlier state newspaper Legal Daily reported another forced labor case, also in Yunnan, involving 26 people, 10 of which had developmental disabilities.

In 2007, some 570 people, among them children, were freed from brick kilns in Shanxi and Henan provinces in northern and central China, respectively. In the aftermath a total of 29 people were convicted of wrongdoing, and one person was given the death penalty for killing a mentally disabled worker.

The judge who announced the verdicts called the phenomenon of forced labor in brick kilns an “ulcer on socialist China,” according to the BBC. “We must get rid of it,” he said.

Shi Pu, a management professor at Henan University of Finance and Economics who focuses on labor issues, told Sixth Tone that there are several reasons behind the phenomenon of slave labor. The government’s support for disadvantaged social groups is not robust, and the country’s law enforcement is not effective at tracking down missing persons, he said.

Shi added that there are some lower-level government officials who collaborate with the kilns. In the 2007 case the owner of one of the brick factories was the son of a local official. He was sentenced to nine years in prison.

“This is not a new phenomenon,” said Shi. “It’s been happening for more than 20 years.”

This article has been updated to include Shi Pu’s comments.

Additional reporting by Li You.

(Header image: Living quarters for mentally disabled people forced into hard labor at the Sheng Heng brick kiln, Yangchang Township, Yunnan province, Sept. 4, 2016. Peng Yu/Sixth Tone)