Authorities have intensified an investigation into a collapse at a gypsum mine in Shandong province after four miners trapped more than 200 meters underground for 36 days were rescued on Friday.

They are also preparing to repair land damaged by gypsum mining.

A team comprising experts and government officials was set up on Saturday to investigate the cause of the accident and to help with the search for those still missing, the local government said.

The four survivors rescued on Friday were trapped when the collapse occurred at a mine belonging to the Yurong Trade and Commerce Co in Pingyi county on Dec 25.

The four, who were spotted by life detection systems on Dec 30, had been provided with food and water. Eleven other survivors were rescued within a day of the accident occurring.

As of Sunday, one worker was confirmed to have died in the collapse and 13 had not been located.

Zhang Shuping, the mayor of Linyi, the city that governs Pinyi, said: “We have never given up search and rescue efforts for the missing 13. We are organizing experts to come up with new rescue solutions.”

Zhang said the authorities will inspect all 212 non-coal mining companies in the city and ban those with safety risks from operating.

He also said the city will organize professional teams to repair more than 1 million square meters of land affected by mining to prevent collapses from happening.

The four miners were in stable condition after being transferred from intensive care wards to regular wards on Sunday, China Central Television reported.

Wu Dawei, an intensive care unit doctor at Shan-dong University’s Qilu Hospital, said the four miners must remain under observation after being underground for so long.

Guan Guozhong, the son of Guan Qingji, one of the four workers rescued, said the past month had been “torture” for his family.

With rescue work and an investigation continuing, farmland at the rescue site and in nearby villages is still subsiding. In some areas, the land has subsided by several meters.

Deep cracks can be seen on roads and some houses near the site, while a hole several meters in diameter and about 10 meters deep lies on farmland nearby.

“The land has subsided so much,” one villager said. “I think the government will have to repair it so we can grow crops on the land again.”

Gypsum mining has been an important source of income for Wanzhuang, a village near the scene of the collapse, according to a report in Jinan Times, a local newspaper.