MONTERREY, Mexico – Four workers were killed Wednesday in an explosion at a power plant in the northern Mexican state of Coahuila, authorities said.

The blast occurred at the coal-fired Carbon II plant in Nava, a town on the highway leading to the border city of Piedras Negras, a state government official told Efe.

The four workers were doing maintenance when a soldering tool make contact with flammable material, causing an explosion.

The victims were burned so badly that DNA tests will be needed for conclusive identification of the bodies, the Coahuila Attorney General’s Office said.

Operated by federally owned utility CFE, the Carbon II plant is located in a region of Coahuila that produces around 80 percent of Mexico’s coal and is the scene of frequent and often fatal mining accidents. EFE