The grainy dash-cam video dates itself: 20 October 2014. The time code shows it’s just minutes before 10pm and the headlights of the police car illuminate the street. Called to the scene to respond with Taser backup to a dazed boy walking through a truckyard carrying a knife who was breaking into cars, the car finally pulls up. In the middle of the street, back turned to the camera a young black male walks hurriedly, seemingly trying to avoid something. He turns slightly, almost as if he has decided to walk backwards. Then suddenly, he drops, shot. Lying on the ground, the pixelated footage doesn’t show the 15 bullets about to pierce his body that will result in his death but there is an occasional puff of smoke, betraying the concealed impact of the fatal projectiles. There, in the middle of the street on the south side of Chicago, 17-year-old Laquan McDonald dies, shot 16 times in 15 seconds by the Chicago police officer Jason Van Dyke, who had just arrived six seconds before discharging his weapon. He would later claim he was in fear of his life when Laquan lunged at him, though the video shows no such action.

Those fatal shots started a landmark four-year civil rights fight which, by its end, exposed Chicago politics and became the basis of the new documentary 16 Shots. The film, written and directed by the Oscar-nominated director Rick Rowley, tells the story of McDonald’s murder and the city surrounding him. It documents the aftermath of the murder: the slow media coverage, the mounting tensions and the heart-quickening anxiety associated with near-certain implosion. Almost immediately, it immerses the viewer in a tale of two cities: one ridden with an angst stemming from injustice and the other refusing to relinquish control and accept responsibility, offering a candid look into the thoughts of activists, prosecutors, former police officers and even eyewitnesses of the murder.

Jamie Kalven, a journalist who had written about the story, briefed Rowley on the topic soon after the release of the video. At the time, it had already been released to the family who were under a gag order not to share it further and was only forced into the public by the activist Will Calloway and the civil rights attorney Craig Futterson. Though no one knew it, this action would change Chicago, toppling heads of citywide institutions and ushering in a deluge of change for the midwestern city. It’s as the former Cook county prosecutor Anita Alvarez says in the film: “It was a tsunami that I certainly couldn’t stop.”

But for Rowley, the murder of Laquan presented a unique opportunity to show the uncoiling of a system that insulates itself against punishments of wrongdoing. He told the Guardian he saw the chance to explore a tragedy too long plagued by the shadow of political institutions. “America’s in the middle of this kind of reckoning,” he said. “I mean, race has been this blood knot at the center of the American experience for 300 years and sometimes, it’s more visible and sometimes, it’s less visible but right now, these struggles around race and violence are coming to the surface everywhere. We have a string of police shootings that have upended our whole discourse around the criminal justice system. This case cuts to the heart of that entire nightmare. We know more about the killing of Laquan McDonald than is known about any other police shooting in history … We have the ability through this to paint a forensic portrait of the kind of system that makes these cases disappear.”

At the crux of 16 Shots is the struggle for accountability. Before the release of the video, the silence fostered by the police department, the prosecutors and the mayor hushed the case, attempting to maintain order under the guise of peace. Even after the release of Kalven’s article and a $5m settlement to the McDonald family, there were few people truly paying attention. “At every point in this process, everything works in support of the official narrative,” explains Kalven in the film. “The machinery is actually continuing to function. Until, dramatically, almost cataclysmically and mysteriously, it doesn’t.” Even Rowley and the team behind the documentary fell victim to the system’s code of silence, with the Chicago mayor’s office refusing to speak, restricting their subject list, and even at one point, blocking the film-makers from taking equipment into public events. While he insists these blocks stopped nothing, Rowley was surprised. “Just the incredibly insular nature of that was sort of a shocking thing to see,” he said.

In sharp contrast to the city, 16 Shots aims to be as transparent as possible, even down to the most horrifying of details, like the actual murder. The video is shown in its entirety in the film, with current prosecutor Kim Foxx commenting on the verge of tears at its heart-wrenching contents. Many dispute the decision to share the video to a wider audience, as it might promote the desensitization of black death. Rowley freely admits it is not something done lightly, telling the Guardian: “We framed it in a way that we hope has as much humanity and dignity as possible.” Being a former war reporter, he has often dealt with the representation of death and wanted to be as humane as possible to the victim. “Our rule is always that, when there are moments of death, you need to deal with them with as much humanity and tenderness as is possible to make it not just another picture,” he said. “Kim Foxx allows us to do that. Because she contextualizes this in a completely human way.”

As seen in the film, the release of the video triggered the battle for a criminal trial against Van Dyke. During the four-year period of limbo between the murder and the final verdict, the city ousted the chief of police, Garry McCarthy, and the state attorney, Anita Alvarez, who later spoke to the film-makers candidly about their removal from office. Rowley simply puts it: “There are revealing moments in those times of rupture.” While they saw it as an assault on their livelihoods, the city’s activists viewed it in a completely different light, chanting “Two down, one to go!” To the activists seeking justice for Laquan, this was a necessary step in the right direction. After all, in its history, Chicago had never indicted or charged an on-duty police officer for the murder of anyone, let alone a black person. The magnitude of the situation was not lost on anyone on either side of the conflict, as the film shows. In the words of Futterman, “This was historic.” For some, like activist Charlene Carruthers, it was a David and Goliath-esque victory. “Heads rolled with this trial,” she said. “Heads rolled with this verdict. And they’re continuing to roll. We did that. So what else is possible now?”

Rowley believes the unprecedented reaction to the case was down to a number of factors. “It’s a combination of so many things at every step of the way, from individual acts of bravery from the whistleblower and the civilian witnesses to what appears to be some bizarre bureaucratic mistake that released the video to the family,” he said. “On the one hand, what you see in the film is every step of the process that the system uses to make these cases disappear. On the other side, you see every moment where someone steps forward and disrupts that process and there were dozens of people who did that along the way.”

While the subject matter is difficult and sometimes blindly angering, Rowley hopes the film will highlight the importance of citizenry to even those who want to turn away. “I hope that for some people, like me, this film will make them pause a moment before turning the page after reading the police blotter account of another shooting like this,” he said. “And for others, it’s a picture of a four-year long struggle that’s sustained public attention and community pressure that managed to break through this wall of silence that is built around these cases.” He believes everyone is implicated when it comes to maintaining an accountable system: “I believe that we all have a responsibility to grapple with this, these issues. There’s not a major city in America that is not dealing with the questions that are raised by this.”

• 16 Shots will be released in New York and Los Angeles on 7 June and will be available on Showtime on 14 June. A UK date has yet to be announced