ON THE EVENING of March 3, 2010, two Prince George’s County police officers, clad in riot gear and wielding nightsticks, beat an unarmed and unthreatening University of Maryland student named John J. McKenna during rowdy street celebrations in College Park following the university’s men’s basketball victory over Duke. The beating was swift, savage and unprovoked; Mr. McKenna had simply been skipping down the street when he encountered the police who slammed him to the ground and pummeled him. Eight metal staples were needed to close a wound to his scalp.

Afterward the police lied about the incident until a video surfaced that exposed their fabrications; even then, they covered up the officers’ identities for months. At trial, a judge with a possible conflict of interest, which she failed to disclose until questioned about it, showed glaring bias in favor of the officers. In the end, one of the officers got off scot-free; the other was given a slap on the wrist; and no higher-ups in the police department were called to account.

This is a travesty of justice. Let’s take a look at who is responsible.

Start with the Prince George’s police department, which engaged in a conspiracy of silence and coverup. Initially, the police accused Mr. McKenna in court documents of attacking and injuring a mounted police officer; they said his injuries resulted from being kicked by a horse. This was an invention, as the video showed. If Mr. McKenna was guilty of anything, it was of being oblivious.

Yet no one in the police department came forward with the truth or even, for several months, the identity of the officers who beat Mr. McKenna, even though plenty of them saw what happened. (The police, who wore helmets with visors during the beating, were identified to prosecutors only when a new chief, Mark A. Magaw, took office and cracked the whip.)

Even after the video surfaced, and even after the county paid Mr. McKenna $2 million in damages after he filed a civil suit, the officers who beat him continued receiving their pay for more than two years while on administrative leave. The officer convicted of the beating, James Harrison Jr., was allowed to retire last month — with his pension. Under state rules, he will not be allowed to work again as a police officer in Maryland.

Then there was the unprofessional conduct of the Prince George’s County Circuit Court judge, Beverly J. Woodard, who presided at the officers’ trial. Ms. Woodard was previously married to a Prince George’s police officer who himself was convicted in 1988 for repeatedly shooting an unarmed suspect in the back, as he lay on the ground, with a confiscated BB gun. When she was assigned to handle a trial involving a similar case of alleged police misconduct and assault, she should have at least disclosed that fact, if not recused herself. She did neither.

She withheld the information until asked about it during the trial by a journalist, Brad Bell of WJLA-TV. At that point she summoned counsel for both sides into her chambers and, in an emotional meeting, refused to declare a mistrial, insisting that she could be fair. Her reasoning remains unknown because she did not allow a court reporter into the meeting.

Nonetheless, Judge Woodard’s comportment on the bench left no secret about her leanings, according to several courtroom observers. During the trial, she was overtly sympathetic toward the defense and hostile to the prosecution, signaling her disdain with facial expressions and by turning her back on the prosecutor, Joseph Ruddy, during his closing argument. Those gestures may have been noticed by jurors.

This month at the sentencing, where Mr. Harrison faced up to several years in prison for his conviction for second-degree assault, the judge dispensed a token punishment: 30 days of home detention, followed by 18 months of unsupervised probation. The judge opted for leniency despite the fact that Mr. Harrison displayed no remorse for the beating or the coverup, and he failed even to acknowledge Mr. McKenna, whom he had bludgeoned with his baton, in the courtroom.

In fact, Mr. Harrison had the gall to suggest he had been persecuted for what he regarded as reasonable use of force. One need only watch the videotape to disprove that self-serving account.

At the same time, Judge Woodard, in her comments at sentencing, went out of her way to vilify Mr. McKenna by dwelling at length on the unruly behavior of some, though not most, Maryland students following the game. She thereby managed to smear an innocent victim by associating him with the unrelated actions of others. Judge Woodard did not respond to phone calls to her office.

Other Maryland students were roughed up and badly injured by the police after the basketball game. At least three were knocked unconscious; two of them required medical care. Nine students (in addition to Mr. McKenna) received a total of $1.6 million in settlements from the county stemming from police violence.

In the absence of video evidence in those cases, the officers who used Maryland students as punching bags faced no disciplinary consequences. Amazingly, the police department’s internal affairs division, which handled the abuse complaints, did not even interview most of the students who were injured. It follows that if no video had surfaced of Mr. McKenna’s beating, that too would have been swept under the rug of police impunity and official indifference.

This cannot be allowed to stand. It sends a terrible message to the Prince George’s police, who now know that consequences for their misconduct, if any, will be slight.

At this point, the only way to redeem this case is for the Justice Department to step in and bring criminal charges against the police for violating federal civil rights laws. Then, and only then, will police in Prince George’s take notice that unprovoked beatings will not go unpunished.