There's more than a whiff of Joshua Oppenheimer to Everardo González's haunting documentary Devil's Freedom [La Libertad del Diablo], which explores the violence endemic to Mexico's drug trade from the viewpoint both of its victims and its perpetrators. It's a premise not outlined as the film begins, emerging instead as it unfolds and the combined weight and content of spoken testimonies gradually contextualise the events being described. Initially people talk only of watching family members forcibly taken away, of having their lives threatened, or their frustration at the failure of the police to take action. Terrible crimes have clearly been committed, but by whom and for what purpose? All becomes clearer over time as the jigsaw pieces fall subtly into place and we are able to draw connections between a mother who has lost her two sons, a man who was kidnapped and tortured, and a young man who recalls committing his first murder at the age of 14.

Cinematically, González takes a resolutely minimalist approach, constructing his film almost entirely from interviews and without the usual crutch of captions, re-enactments or photographic evidence. His intention is clear, to focus our attention completely on the experience of both the victims of the violence and those who dish it out. In a decision that may have been initially born of the desire to protect interviewee anonymity, the faces of all of these interviewees are concealed behind identical, flesh-coloured fabric masks with holes for the eyes, mouth and nose, like featureless versions of those worn by Mexican Lucha Libre wrestlers. The effect is unsettling from the start and becomes increasingly so as the film progresses, stripping whole families of their facial identity and subtly suggesting that the paths to becoming a cold-blooded killer or an unwitting victim both have their origins in the same streets and neighbourhoods. Occasionally, the masks themselves even play a narrative role, as in the static, pre-interview shot of one female subject whose heavily tear-stained facial covering acts as a flash-forward warning of the distressing nature of the story she is about to tell.

Initially, the focus is on the victims of the violence, but it's when González moves onto those who commit it that the film is at its most disturbing, notably the young man who describes his first kill in the manner of teenager recalling a lively night out and reflecting on his lack of emotional involvement in an act that all too quickly became an instinctive and everyday part of his identity. This is at its most chilling when he talks about executing the families of men who have fled rather than face the wrath of the cartels – "The hardest part, even if you don't have feelings, are the children," he states in a manner that gives him pause for thought but seems unlikely to have given him many sleepless nights. Tellingly, when asked what he gets from killing someone, the same individual responds instantly with a single word: "Power."

Despite the use of featureless masks, it quickly becomes easy to distinguish the interviewees from each other and recognise them when they reappear to bring previously unfinished stories to their unhappy conclusions. We are also given a sense of the influence and reach of the cartels through the testimonies of policemen and military personnel, who far from just being paid to look the other way appear to have even been instructed to carry out hits by the cartels for pathetically petty rewards. "We're specialists in taking orders," one soldier notes, adding, "It used to be $10 for taking a life."

The interviews are abstractly chaptered by static shots of the local landscape, all of which have been graded to create the sense of a district or even country that is living under a sinister pall, strongly emphasised by composer Quincas Moreira's deeply disconcerting, almost avant-garde score. González is not presumptuous enough to start offering solutions to such a long-standing and complex social problem, but in personalising the violence and the fear it inspires and exploring through direct testimony how it affects lives, families and even communities, his conceptually simple but compelling film quietly packs a cumulatively devastating punch. Bleak it may be, but the film's intentions are purposefully and movingly realised, and it ends on a small but uplifting act of symbolic defiance that left this viewer in no doubt about the continuing strength and spirit of those who have suffered but absolutely refuse to knuckle under.

Devil's Freedom is screening at the BFI 2017 London Film Festival on the following dates:

Thursday 12 October 2017 20:45 – ICA Cinema, Screen 1

Friday 13 October 2017 14:45 – Vue Leicester Square, Screen 7