Three construction workers who were working in a north China coal mine choked to death early Friday morning, local authorities said.

The accident occurred at about 2 a.m. in a coal pit of Shanxi Yangquan Coal Industry Group, Shanxi Administration of Coal Mine Safety said in a statement.

An initial investigation indicated that the three workers had entered an unventilated pit tunnel, according to the statement.

All three workers are from a construction company affiliated with the mine group, the statement said.