Members of the Zetas drug gang used a prison in northern Mexico as their private house of horrors where they tortured and killed kidnapping victims and underworld enemies, public prosecutors in the state of Coahuila have said.

Between 2009 and 2012, Piedras Negras prison became a virtual extermination camp, ruled with impunity by the notorious crime cartel as an operational base for their reign of terror in the US-Mexican border region.

After an investigation into the three-year period, authorities estimate that around 150 people were murdered inside the prison, with their bodies being burnt or broken down in acid-filled tanks before the remains were disposed of in a river some 20 miles away from the jail.

It is not clear to what extent the prison’s official guards actively cooperated with gang members or whether they merely allowed them to act with impunity in exchange for keeping order among inmates.

But prosecutors have revealed that Zetas’ prison leaders dressed up in uniforms as the prison’s de facto security force, wearing bulletproof vests and driving customised vehicles.