Reforma newspaper says the Zetas produce or buy 10,000 tonnes of coal a week. Selling it at their inflated prices, that means yearly revenue of $22 million to $25 million.
Moreira says the drug lord had his own coal pit in the region.
Highway 57 heading north to the United States runs through a dusty black area where piles of coal from small, precariously operated mines dot the landscape. Fatal accidents are common.
Trucks loaded with coal are stopped at checkpoints manned by soldiers looking for drug traffickers and drug shipments.
Since the Zetas discovered coal, violence has been on the rise, especially in a town of 150,000 called Piedras Negras, or black stones.
In Colombia, for instance, traffickers infiltrated gold and coal mines and also dealt in oil.
"Corruption is their main tool for doing business, and also violence, if necessary," Mazzitelli said.
Such business activities allow them not just to bring in more money "but above all gain social and political legitimacy," Salcedo said.
Traffickers want to be able to "legalize their leaders and activities and join the formal economy, and be able to operate in society in a more relaxed way," he explained.
But that quiet end does not always involve peaceful means.
Traffickers sometimes kidnap, mug or even kill miners and their bosses, or force them into business-sharing agreements, said Salcedo.
In Coahuila, some companies without mines or employees have contracts with local coal industry promoter Prodemi, according to a researcher from a local organization founded by relatives of miners who died in a 2006 accident that claimed 65 lives.
"There are mines that have a capacity for 30,000 tonnes but have contracts for 150,000. What they are selling is not what they are producing," added the researcher, who requested anonymity.
"They are buying it from a third party and that is where all these people come in, be they Zetas or not, legal or not, clandestine or not."