Mexican authorities are investigating three coal mining firms for their alleged links to drug traffickers, a Mexico City daily reported Wednesday.

Citing unidentified officials, the Reforma newspaper said the Los Zetas drug cartel, regarded as Mexico's most violent, was the gang allegedly involved in this business in the northern state of Coahuila.

The article was published a week after former Coahuila Gov. Humberto Moreira said groups of "narco-miners" were extracting coal in that state and selling it to intermediaries.

Moreira, former chairman of the Institutional Revolutionary Party, or PRI, which will take power on Dec. 1 after a 12-year hiatus, made the remarks last Thursday after his son, Jose Eduardo Moreira Rodriguez, was killed by Zetas gunmen.

In subsequent statements, Moreira said that in the coming days he would provide details on these and other allegations.

According to Reforma, illegal mining generates at least 6 million pesos (some $460,000) in weekly income for organized crime gangs, which later launder those proceeds through legally incorporated entities.

According to the daily, the firms Impulsora JBN, Perforaciones Tecnicas Industriales and Minera La Mision are under investigation "for their alleged role in these illegal businesses."

The mineral resources are sold to state-owned electric company CFE, the daily said.

The sources told Reforma that the Zetas' point man for this business is Jose Reynol Bermea, who runs several coal mines in Coahuila.

Efe was unable to reach the CFE for comment. EFE