Family annihilators, or familicide perpetrators, are one of the last frontiers of true crime literature and television. Their motivations are often obtuse, they frequently vanish or commit suicide in the wake of their crimes, and their actions are often unpredictable – at least from a distance. Jenny Popplewell’s documentary, American Murder, addresses this situation through Chris Watts’ murder of his wife and two children in Frederick, Colorado, in late 2018.

The genius of American Murder is that it is entirely comprised of prerecorded footage, meaning Popplewell’s role as a director mainly consists in curating, compiling and juxtaposing these images. The film centres around the last recorded image of Shanann Watts, who returned home from a holiday on August 13, 2018, and was last filmed from her own doorbell camera as a friend who had picked her up from the airport drove away. From here, Popplewell moves backwards and forwards at once, but never strays too far from this last known sighting.

Moving backwards, we’re introduced to Shanann as a prolific Mommy Vlogger who apparently shared every detail of her personal life on Facebook. Popplewell doesn’t need to provide any commentary or context, so comprehensively does Shanann outline her own backstory in her Facebook posts, many of which occur in the rooms and spaces that were scoured immediately after her disappearance. We’re also given access to her many text messages with friends, all of which document her growing unease and discomfort with Chris.

Moving forwards, Shanann’s posts are intercut with the original canvassing of the Watts House, which was all recorded on police chest cam. We see the entire first morning spent at the house, from the moment the police arrived (called by Shanann’s friend), to their first interactions with Chris, their first search of the house and their first interviews with neighbours, one of whom confidentially tells them that Chris is clearly acting out of character.

This is the most incredible part of the film, since these early moments in the investigation, which are so often recalled or reconstructed after the fact, are captured entirely on camera here. We’re present for the inchoate dawn of the case – the point where the police are just suspecting a crime might have occurred – and immersed in the eerie emptiness of the house while it still bears the immediate psychic traces of the crime. There’s only the slightest ambience of foul play, and only the slightest strangeness to Chris, but it’s infinitely creepier than anything that follows, just because it’s still – almost – entirely embedded in normality.

So scrupulously is everything captured, and so neatly does it all come together – especially footage taken from the suspicious neighbour’s garage cam – that this almost feels like a fictional film, which of course makes the documentary quality even eerier. The result is like watching the recent desktop thriller style transplanted to true crime, as Popplewell cuts between Facebook, security footage and news broadcasts with a deft rhythm that feels particularly drawn from Aneesh Chaganty’s Searching, which was also a missing persons narrative. The austerity and brevity of American Murder – its total commitment to found footage – also reminded me of The Blair Witch Project, in its own way also a vanishing story.

It’s only a matter of time before Chris becomes the prime suspect, so the second half of the film revolves around the polygraph and police interrogation, which is also recorded in its entirety, and elicits the revelation that Chris was seeing a younger women in the months before his wife and daughters’ disappearance. Popplewell deftly pre-empts the victim-blaming that typified this case, presenting us with Shanann at her most demanding and difficult – shaming and blaming Chris on social media – to contextualise the way that the media turned against her after Chris attributed the murder of their young daughters to her.

For a brief moment, it does feel plausible that Shanann might have been responsible in some way. Yet this impression quickly fades, as it did for the public at large, thanks in large part to how eloquently Popplewell curates Shanann’s messages and Facebook posts in the final week before her death. We follow Shanann’s dawning awareness that something isn’t quite right about Chris’ empathy, which he seems to be “performing” in the way that sociopaths typically do. Experiencing these last social media texts is like witnessing someone recognise a sociopath in real time, and makes Shanann’s actions more relatable – as if she’s obsessively recording and oversharing to convince herself that Chris is properly, emotionally attuned to the world around him, or in an effort to elicit some authentic emotional response from him.

Yet the more she pushes, the more he responds with pat, trite, preprepared gestures of sympathy. These start off seeming conciliatory, but quickly become creepy, turning the last part of the film into a portrait of his sociopathy – or an evocation of how inscrutable his sociopathy remains. For all that Chris eventually confesses to the crimes, he can’t articulate why he did it – and yet even his tortured dismay at his own actions feels off, as does his timeline of how and when the murders took place, and his account of his psychological state.

There seems to be some consensus amongst forensic psychologists that family annihilations are nearly always premeditated events, due to the extremity of the crime, but also the logistical challenges of making sure it all goes to plan. Chris Watts doesn’t seem like an exception to that rule, so seeing him lament his rash judgement just add to his eeriness, much as the transparency and procedurality of Popplewell’s found footage style makes this form of crime seem even more remote and alien by the closing sequences. The result is possibly the best documentary made about family annihilation and a brilliantly experimental film on its own terms that just may signal the start of a new subgenre in the current true crime revival.