Yesterday The New York Times published a video showing a police officer, Michael T. Slager, fatally shooting a black man, Walter L. Scott, as he ran away from the officer.

The video is disturbing enough by itself. But it becomes even more troubling when we consider how radically at odds the visual evidence seems to be with the police incident report filed on the killing. As the Times notes, Slager “said he had feared for his life because the man had taken his stun gun in a scuffle after a traffic stop on Saturday.” Yet the video shows Scott killed in flight, something like 20 feet away when the final bullet hit. After the shooting, Slager is shown placing an object next to Scott’s prone body. According to the Times, police reports also claim that officers performed CPR on Scott, an assertion not borne out in the video.

The death of Walter Scott will add more tinder to the already blazing political debate over police violence. The apparent contradictions between the incident report and the video highlight an overlapping but distinct problem: The police don’t always tell the truth. Police violence and police lying are two separate problems, although they also reinforce each other. Police violence flourishes in part because of the prevalence of police lying, which is rarely challenged by the criminal justice system.

In the Scott killing, there is good reason to believe that without the powerful counter-evidence provided by the video, which led to Slager being charged with murder yesterday, the police incident report would have been accepted as the official account of the shooting. Indeed, the persuasive power of police testimony extends outside official channels. Prior to the emergence of the video and Slager’s arrest, Slager’s version of events was echoed by the local media in South Carolina as if it were factual.

Police lying doesn't just act as a shield for police violence, but as a larger source of corruption in the criminal justice system. Criminal cases are always narrative battles: Prosecutors and defense attorneys compete to win cases by presenting the most plausible stories consistent with admissible evidence. The police play a crucial part in this system as a supplier of narrative facts, in the form of both reports and testimony under oath.