An explosion on Wednesday in a coal mine in northern Mexico killed seven miners, officials said, highlighting lax safety conditions in small mines that are often poorly regulated.

Emergency services brought the bodies of the men, aged between 20 and 39 years old, to the surface after an hours-long rescue effort, said Juan Antonio Ibarra, a emergency services official in the coal-mining state of Coahuila.

“Sometimes these mines don’t have the appropriate security measures,” said Ibarra.

Ibarra said a pocket of gas likely caused the explosion that occurred 246 feet (75 meters) underground.

The rescue operation, which involved up to 50 people, had moved slowly because of the need to ventilate the mine to prevent another explosion, Ibarra said.

The “pozito,” or little hole, coal mine, which is privately operated by the El Progreso mining company, faced fines and sanctions in past, the Mexico’s labor ministry said in a statement.

The mine had been inspected 16 times since 2009 and access restrictions were enacted as recently as June after inspectors found it lacked emergency exits, the labor ministry added.

“The government pledges to continue inspections of high-risk mines across the country,” the statement said.

The company could not immediately be reached for comment.

Men have mined the largely unregulated pozito mines that dot Coahuila for more than a century.

A 2006 methane explosion at the much larger Piedras Negras mine, owned by Grupo Mexico, killed 65 miners.