Police discovered the documents on computers seized during operations against the National Liberation Army's (ELN) Dario Ramirez Front in the northern department of Bolivar, reported El Tiempo.

The documents detail the guerrillas' involvement in mining in the region, including a breakdown of their extortion fees, and regulations imposed on local miners – listed as seven "rules of the game."

According to the files, 50 percent of local mining profits should be directed to the ELN front. The documents also mention "a small gold reserve that is going to be invested in," and give instructions on "holding reserves."

On another computer, police found accounts related to the ELN's extortion activities, including payments collected from cooperatives and other associations in the region, as well as extortion fees on use of equipment such as bulldozers -- which miners must pay the ELN approximately $50 an hour to use.

InSight Crime Analysis

Criminal and rebel groups have long been involved in illegal mining in Colombia, and it has become the principal source of criminal income in certain regions.

Previous reports suggested the ELN were primarily focused on charging miners an extortion fee for the right to work in their territory, as well as another fee in order to bring mining machinery into areas under guerrilla control. The documents referenced by El Tiempo suggest the ELN is now following its larger cousins the Revolutionary Armed Forces of Colombia (FARC) by expanding into direct control over mining activities to extract greater profits.

The files also suggest the ELN are following a similar trajectory in the illegal mining sector to the path they took with drug trafficking -- where again they were some years behind the FARC. Initially the ELN refused to enter the drug trade, before later charging "taxes" on traffickers operating in areas under their control, then finally becoming more directly involved after striking alliances with drug trafficking organizations such as the Rastrojos.