IF A WITNESS hadn’t shot video of two Prince George’s County police officers savagely beating John McKenna, a University of Maryland student, after a March 2010 men’s basketball game, that would probably have been the end of it. The officers didn’t file a report, as required, on their use of force. When initially questioned about the beating, they lied. And when they filled out the initial paperwork on the incident, police said Mr. McKenna had sustained his injuries, including a concussion, from being kicked by a police horse.

Thanks to the video, and to the persistence of Mr. McKenna’s lawyers in uncovering it, the coverup didn’t work. This fall a Prince George’s jury convicted one of the officers, James Harrison Jr., on a charge of assault. The other officer, Reginald Baker, was acquitted, although he, too, used his baton to beat Mr. McKenna as he lay stunned and defenseless on the ground.

On Friday, Mr. Harrison was sentenced, too lightly. Although he faced up to 10 years in prison for the second-degree assault, and prosecutors asked for six months of prison time, Judge Beverly J. Woodard gave Mr. Harrison little more than a slap on the wrist: 30 days of home detention, followed by 18 months of unsupervised probation.

The sentence sends the wrong message — that even if police flail away at an unarmed and non-threatening student; even if they leave him with a concussion; and even if they lie about it until forced to devise an explanation when video footage surfaces, they will serve no prison time. That’s wrong.

In the video, Mr. McKenna, one of hundreds of students milling about in College Park in an unruly celebration, is shown skipping up to riot police on their horses. He represents no threat, clearly means no harm and takes no aggressive action. He stops and begins to step away from the police. In the next instant, he is rushed by officers on foot, wearing riot gear, who slam him to the ground and beat him with their sticks. The beating goes on for about four seconds, enough time to hurt him badly. And then the police close ranks, driving other students away.

There were dozens of witnesses, including police. But inside the police department, investigators were met initially with silence. It was only when the video appeared that the wall of denial began to crumble.

According to the Prince George’s police, Mr. Harrison left the police force at the end of November. His real punishment is disgrace; under state rules, he is ineligible to work again as a police officer in Maryland. Mr. Baker’s fate remains unclear. He remains suspended with pay pending completion of the police department’s internal disciplinary process.

Mr. McKenna is reported to have received a $2 million settlement from the county. But when police act without discipline and then cover it up, they forfeit any claim to credibility and trust. It’s a pity the judicial system did not take the offense more seriously.