In May 2010, New Jersey single mother Mari Gilbert reported her eldest daughter Shannan missing when she failed to turn up for a family dinner. The initial disinterest and disdain she encountered from local law enforcement persisted through a subsequent investigation – fuelled by Mari's unending protests at the lack of action – which resulted in the discovery of the remains of several women, all victims of a (never caught) Long Island serial killer.

This extraordinary case is the focus of the narrative feature debut from prolific, double Oscar-nominated documentarian Liz Garbus (The Farm: Angola, USA, What Happened, Miss Simone?), who brings both authenticity and intimacy to this sensitive story. Crucially, she upends movie tradition to focus not on the police officers at the centre of this bungled operation, but squarely on Mari and the group of women – mothers, sisters, daughters of the dead – who grow up around her, drawn and motivated by her persistence and determination to keep her daughter in the spotlight.

Amy Ryan is mesmerising as Mari, the hard-bitten exterior hewn from years of tooth-and-nail struggle – she works two jobs, can only just afford to live – barely concealing the tumult of fear and grief under the surface. But while she may be suffering, she is no victim; Mari digs in her heels and uses everything at her limited disposal to fight for her daughter. That Shannan and the other victims were all working as prostitutes gives the police licence to treat them, and their families, like second class citizens, unworthy of justice. Mari not only calls them out on it, but makes sure their bias is exposed for the world to see.

It's a difficult watch, but Garbus avoids melodrama or soapboxing to instead focus on the horrific truths about the issues of class, crime and the abuse of power that were exposed by this case. She's helped by evocative lensing by cinematographer Igor Martinovic, who captures the claustrophobic social confines of blue-collar America and the sombre slog for justice. The camera, like Michael Werwie's measured screenplay (adapted from the book by Robert Kolker), remains trained on Mari and the other women who have been ill-treated by the system, giving both them and their lost loved ones the attention – and voice – that they deserve.

Available to watch on Netflix now.