Prolific documentarian Nick Broomfield (Aileen: Life and Death of a Serial Killer, Battle for Haditha) turns his unique sights on the grisly case of the infamous serial killer known as the Grim Sleeper, who terrorized South Central Los Angeles for over a period of 25 years, killing as many as 100 women, before prime suspect Lonnie Franklin was finally arrested in 2010.

Where a lot of filmmakers would take a more distant, maybe even cold approach to such a horrendous subject matter, Broomfield digs right in there, frequently featuring on camera with a microphone in hand trying to get every morsel of detail he can about the case and all those involved, however tangentially, from direct survivors of the killer’s heinous acts to passers by who may or may not have heard a story a few years back.

It’s as much a fish-out-of-water culture clash film as it is one about gruesome crimes and social injustice. Broomfield (someone who couldn’t be anymore different from the people he’s interviewing) wanders the streets of the area in which the crimes took place, talking to those who knew Lonnie – both those who suspect his guilt and those who vehemently protest his innocence – and he makes you feel like you are as much a part of the investigation as he is.

Although the story feels incomplete by nature – the case is still ongoing – it’s a documentary that feels like it gives you the most complete picture possible given the available evidence and (conflicting) reports about what exactly happened. He paints a fascinating portrait of a downtrodden yet extremely close-knit community and how such a terrible on-going and unjustly glossed-over crime epidemic affects those who lived in it during the killer’s reign of terror and the deep emotional scars it has left on the victims’ family and beyond.

As well as being an investigation into the murders themselves , the film also explores the daring idea that the police either didn’t really want the case to be solved or put less effort into getting things done because of the poorer, non-white section of the LA community that the murders affected. A fascinating and rather disturbing commentary by one of Broomfield’s interviewees mentions certain officers treating Lonnie like a celebrity because of his supposed “cleaning up” of the streets of victims who happened to be prostitutes and drug addicts. He admirably and boldly gives a voice to people that were otherwise ignored.

It’s not exactly what you would call easy viewing, particularly when it comes to seeing certain photographs of the victims and explicit descriptions of what happened to them, but nor should it be. It’s a film that skilfully builds a bigger picture through carefully observed and chronicled details, one that you won’t soon be able to shake from your mind. This is a sobering, eye-opening, often exasperating documentary filmmaking with the power to drive at the heart of both its difficult subject matter and the heavy emotion that it brings with it.