An explosion in an illegal underground gold mine near Continental Gold’s Buriticá project in Antioquia, Colombia, killed six of the company’s security contractors on July 28. 

The contractors were performing “routine underground inspections” of nearby illegal mines closed by the Colombian government when they were confronted by illegal miners, “followed by a subsequent explosion,” the company said. The contractors died from asphyxiation.

According to Inka Kola News, the attackers set off explosions and also burned tires, and the contractors died from asphyxiation from the resulting smoke. IKN reported a seventh contractor survived the attack by ducking into a side tunnel, and was pulled out by a rescuer.

“We are deeply saddened by this terrible tragedy and our thoughts and prayers go out to the families, friends and colleagues of the victims,” Continental CEO Ari Sussman said in a news release. Continental said it is working with government agencies to support the victims’ families.

After the explosion, rescue efforts were led by Colombia’s National Mining Agency and the government’s disaster recover agency, along with help from Continental.

Illegal, or artisanal, mining is one of the challenges faced by mining companies looking to operate in Colombia, and one that Continental has been grappling with at Buriticá. According to the company artisanal miners were accessing veins the company had defined, and their mining methods polluted the water at the site and in nearby communities. They also spread negative sentiments to communities about Continental’s project.

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The Colombian government stepped in to close illegal mines and resettle the artisanal miners in 2016 with the help of a third-party NGO to make sure they had food, water and psychological support. “The intervention effort…was successful in closing the vast majority in an orderly and peaceful manner,” Continental said in the same release, adding however that “a few illegal mines continue to operate.”

Security contractors have been mandated by the government to inspect closed illegal mines as part of its post-intervention strategy.

Continental urged local communities and the state and national governments to “unite on this tragedy and pursue all legal means under the law to bring the perpetrators to justice.”

With files from Sahar Fatima