A Baltimore police officer who was caught on camera repeatedly punching a pedestrian who dared to question his authority resigned yesterday after his department suspended him and launched an investigation. The swift response to the ugly incident, which happened around noon on Saturday, shows once again the importance of ubiquitous cameras in revealing and deterring police brutality, which may be partly a function of race but is fundamentally a problem of unconstrained power. That truth is especially apparent in this case, where both the abusive officer and his victim are black.

The Baltimore Police Department has not named the officer who resigned or the man he assaulted. But Warren Brown, an attorney who represents Dashawn McGrier, the man who was beaten, identified the officer as Arthur Williams, who joined the department last year. Brown said the encounter on Saturday, which was recorded by more than one bystander, was McGrier's second run-in with Williams, who last June arrested him for assaulting an officer, disorderly conduct, obstructing and hindering, and resisting arrest. All of those are highly malleable charges that look even more questionable in light of what happened on Saturday.

According to Brown, McGrier was sitting on the steps of a building when Williams passed by in his patrol car. Moments later, as McGrier was walking down the street, Williams approached him on foot and ordered him to stop. McGrier wanted to know why he was being stopped, but Williams would not say. "I'm sitting on the steps," McGrier says in one video. "For what?" Williams pushes him, and McGrier says, "Don't touch me!" Williams responds by punching McGrier more than a dozen times and tackling him. McGrier does not fight back. As he lies on the sidewalk under Williams, blood flows from his mouth. "I got all that," says the man who is recording the attack. "Don't worry."

Interim Police Commissioner Gary Tuggle said he was "deeply disturbed" by the videos and had launched an investigation of Williams and the officer who was with him, which will include a review of "body camera footage." The second officer, who like Williams was suspended with pay, did not participate in the assault but did not intervene either. Mayor Catherine Pugh described the incident as "disturbing" and said she had "demanded answers and accountability." She added that "we are working day and night to bring about a new era of community-based, constitutional policing and will not be deterred by this or any other instance that threatens our efforts to re-establish the trust of all citizens in the Baltimore Police Department."

Even Lt. Gene Ryan, president of the local police union, said Tuggle took "the appropriate action" in suspending Williams pending an investigation. "I'd like to believe that there is more to it, but obviously, it really makes us look bad," Ryan told The Baltimore Sun. "That's something we don't need right now. We don't need another black eye."

But for the bystander video, the official response might have been quite different. McGrier, the victim, might have been treated as a criminal instead. Without video evidence to contradict him, Williams could have claimed McGrier resisted arrest for disturbing the peace, or some other combination of easily invented charges.

In its initial description of the incident, the BPD said two officers "working a special cross borders crime initiative…encountered a man, whom one of the officers is familiar with." According to the BPD, "After the first encounter, officers released him and then approached him again to provide him a citizens contact sheet. When he was asked for his identification, the situation escalated when he refused. The police officer then struck the man several times."

Ironically, citizen contact sheets are meant to help prevent police abuse by providing a record of street stops. But McGrier seems to have provoked a beating by making it difficult for Williams to fill out this supposedly protective paperwork. The BPD statement mentions no justification for the initial stop, which was supposed to be based on "reasonable suspicion" that McGrier had committed a crime or was about to do so. A demand for identification under threat of arrest likewise is unconstitutional in the absence of reasonable suspicion. But reasonable suspicion, like criminal charges for people whom police deem insufficiently deferential, is easily manufactured.

"I don't think there was any room for the activity that I saw," Commissioner Tuggle said at a press conference today. But in practice police have a license to harass people at will, inventing excuses as necessary, unless there is video or eyewitness testimony to contradict them. While young black men like McGrier are especially likely to be the victims of such abuse, the basic problem is loose rules and weak mechanisms for enforcing them.

Some people might fault McGrier for responding defiantly to Williams instead of meekly complying, which probably would have saved him a beating. But people who are not reasonably suspected of criminal activity are under no obligation to provide identification, and McGrier was rightly indignant that police were hassling him for no valid reason. In a free society, no citizen should have to fear police punishment for asserting his rights.