The blast occurred at ​the La Ciscuda mine, operated by Carbonera ⁠Los Pinos.

The company could not immediately be ​reached for comment.

The mining agency said the accident ​seemed to be caused by a build-up of gases, and that it had recommended that the mine strengthen ​its safety measures during a site visit on ​April 9 in which it had identified gases, including methane, ‌that ⁠it said could become dangerous.

“As the ANM has warned during its inspection visits, coal deposits can present accumulations of gases such as methane, ​as well as ​concentrations of ⁠coal dust,” the agency said in a statement.

The six workers who ​survived were taken to a regional hospital ​for ⁠treatment.

Serious accidents are common at open-pit and subterranean coal and gold mines in Colombia, mostly at ⁠illegal ​or informal operations and those ​without proper safety measures.