The decline in violent crime in Los Angeles has been among this region's most gratifying and encouraging trends in recent years. But some have worried that the decline must inevitably level off and give way to stasis. Happily, year-end data from the Los Angeles Police Department suggest there is still progress to be made.

Sociologists and criminologists once doubted that police could do much about violent crime. Violence, the theory went, was attributable to any number of social phenomena — deprivation, demographics, drugs, bad parenting — that were beyond the reach of police. The best that could be hoped for in terms of law enforcement were marginal gains and maintenance of public order. Those notions were challenged intellectually by the work of James Q. Wilson and George L. Kelling, and later others, and were upheld by a new generation of policing strategists, most notably former LAPD Chief William J. Bratton.

Social factors, of course, affect crime, but today no sensible student of policing denies the effectiveness of smart, data-driven law enforcement. And the results of that work are apparent yet again in Los Angeles' crime numbers for 2010: For the first time since the late 1960s, the city appears on course to finish the year with fewer than 300 murders. That is a stunning drop from the early 1990s, when more than 1,000 Angelenos were killed by others in 1991, 1992 and 1993.

What may be even more encouraging, however, is that the city's crime statistics suggest further progress is possible. That's because a significant number of the murders and other violent crimes — assaults, rapes, robberies — being committed in Los Angeles are of the sort that traditionally have been considered "repressible," crimes that could be deterred by more or better policing. Repressible crimes are distinct from the type of violence that presumably no amount of law enforcement can prevent, either because of the nature of the offense or because it occurs behind closed doors. Domestic violence probably would be with us even if a cop stood on every street corner at all hours of the day and night; a certain amount of reckless driving or fighting in bars is inescapable regardless of police deployment. But crimes committed in public by people barely known to each other are less likely to happen if the culprit fears apprehension and prosecution.

Of the murders in Los Angeles this year, police estimate that half were gang crimes, and most of those are considered repressible. Smart intervention, focused community programs and thoughtful policing already have produced significant declines in gang crime, and can continue to protect this city from its cost. Gangs may be a permanent fixture of urban American life, but endemic gang violence need not be. Los Angeles is proving it.