THE ANGER and grief stirred by the holiday weekend's rock-throwing incident on the Capital Beltway and this week's arrests aren't fading easily -- not only because of the nightmarish harm that was inflicted but also because of disturbing evidence that earlier citizen complaints were not taken seriously by the Maryland State Police. Even if the deadly consequences of this warped pastime weren't that fully understood until the Oxon Hill tragedy, the reports of apparent police indifference in the past now merit as much attention as authorities have been giving to this latest event.

The police reportedly reviewed five similar rock-throwing incidents that occurred during the past six months. In one of those cases, a woman who got in touch with this newspaper said that her boyfriend took photos of youths who threw a landscaping stone at her car on May 24 near the Livingston Road site of the last event. Several other people called The Post to report similar incidents and to say they felt their complaints had not been taken seriously by the state police. One caller said he was told to take his story to the Prince George's County police; another said when he reported an incident, the state police "accepted it very calmly" and told him that rock-throwing was "very commonplace" on that stretch of the beltway.

If it was or is, that's all the more reason for police to stop it from being "commonplace." What's the difference between assault with a deadly weapon -- a shooting -- and assault with rocks that hit cars at potentially lethal speeds? Why should teenagers think of rock-throwing as something fun to do -- and not as a crime? Why only now are authorities looking into the possibility of fences or barriers near overpasses and at other sites along the beltway? Police have characterized the last incident as the worst in recent memory on the Beltway. They should take all the steps necessary to ensure that it remains the worst.