State media reports that 18 miners have been killed in a mine-shaft collapse in northwest China.

Twenty-eight miners were working underground when the accident happened Tuesday at the state-owned Huisen Liangshuijing Coal Mine in Shaanxi province, the official Xinhua News Agency reported Wednesday.

Twelve men were rescued but two of them later died in the hospital, it said. Rescuers searched for the remaining 16 trapped men but later confirmed they had died.

Work safety officials are investigating the cause of the collapse. China has the world’s deadliest mining industry, with more than 3,700 deaths a year.