Blowing the whistle very nearly cost John Dodson his career and his livelihood. Now, I am told he may be offered a new position, as a Polygraph examiner. Clearly someone in the ATF must have finally realised he is a man who knows a lie when he hears it.

Dodson tells the story of how he and his colleagues had been involved in Fast and Furious, the codename for a secret gun tracing operation that was meant to dismantle Mexican gun trafficking rings. Instead, they found themselves receiving orders to simply watch as thousands of weapons were being trafficked across the border and into the hands of Chapo’s Sinaloa cartel. Among them AK47s assault rifles, AR-15s and a significant number of 50 caliber rifles, firearms capable of piercing through the wall of a house.

Dodson stopped believing the operation was truly aimed at bringing down a criminal organisation and suspected it was rather aimed at arming it.

A leaked official document appears to indicate that at least three of the weapons used in the massacre were part of a US so called gun tracing operation.

BBC2's award-winning This World strand investigates the American authorities' relationship with the biggest and most powerful criminal organisation in the world, the Sinaloa Cartel, a multi-billion dollar international corporation with franchises in 58 countries. Despite its leader's arrest, the cartel is still enjoying extraordinary success, and this programme examines allegations that the group has been given an easy ride in return for informing on other cartels.

High-level informants, immunity deals, government-sanctioned gun trafficking and a mysterious go-between charged with carrying messages between the DEA and the cartel: a picture emerges of a dirty war being fought with little regard to the thousands of victims of the violent conflict being fought for control of Mexico's drug-smuggling routes to America.