“The word ‘vigilante’ has been given a bad name by the media,” laments self-appointed border guard Tim Foley, a recovered meth addict who now heads up a militia that operates along the Mexico/ Arizona border. It seems most unlikely that this superb documentary will do anything to revise that view.

Cartel Land is characterised by access to two vigilantes: Foley on one side of the border and Dr José Mireles on the other. The latter's prime target is the Knights Templar, a notorious criminal syndicate whose activities include people and drug trafficking and protection rackets.

Soon into the movie, we are taken to a mass funeral for workers of a lime farmer who refused to pay up. Survivors tell how the Knights smashed babies against rocks before throwing the bodies down a well. Sensitive viewers will note that Cartel Land features almost as many severed heads as attached ones.

Mireles responds by raising his own a makeshift army – the Autodefensas – who travel from town to town to root out criminals. Despite opposition from the official military (who the doctor will tell you are complicit), everywhere they go, they are greeted by cheering masses who have lost family members, often in gruesome circumstances.

Long before the viewer gets swept up in the momentum, we have our doubts about these enterprises. Early on, Foley notes that he doesn’t necessarily agree with the motivations that have fired others in his group, many of whom seem to exude a whiff of sulphur and rank xenophobia.

In Michoacan, there is a chilling moment when Mireles, stumbling upon a suspected Templar, suggests that while he doesn’t want the movement to get dirty: “They’d do it to you”.

The drama of these frontlines is made all the more extraordinary by director Matthew Heineman’s remarkable access and by breath-taking camera-work. The road to hell, as paved with best intentions, has never looked better.