District Police Chief Cathy Lanier comforts a grieving mother at the crime scene where her son was shot and killed. (J. Lawler Duggan/For The Washington Post)

THE RESURGENCE of big cities, including Washington, over the past couple decades coincided, not coincidentally, with a massive drop in crime. But murders are up nearly 30 percent in Washington this year, including three in the space of five hours on Tuesday. Hidden in the numbers are horrifying stories of lives cut far too short, like that of Tenika Fontanelle, a 31-year-old mother who died from a gunshot wound as her son got caught up in a petty fight over some rock throwing.

Washington isn’t alone: City leaders across the country are struggling to explain surprisingly negative crime numbers. Are the good times over? Has the Ferguson controversy eroded confidence in and among police? Has criminal justice reform gone too far?

Any such conclusion is premature, at best, particularly because it’s unclear whether we are seeing the beginning of unsettling trends or mere blips. In most cities, crime rates remain well below historical highs. For all the recent carnage, Washington probably won’t reach a murder count this year that matches totals from the 1990s or even much of the 2000s, and violent crime is flat. New York City has seen an increase in homicides from last year, but the number merely matches the 2013 level, and there hasn’t been a rise in total crime. In Los Angeles, violent crime is up by 20 percent, led by an uptick in aggravated assaults, but murders are down by 2.4 percent. The extreme cases are places such as Milwaukee, which has seen its murder rate double this year, and Baltimore, which saw its 212th murder on Wednesday, pushing the city closer to hitting the 300 homicide mark, a threshold it hasn’t breached since the 1990s.

If the overall picture defies simple definition, it’s even harder to point to a simple explanation. Baltimore’s deadly summer seems directly related to the controversy over Freddie Gray, an African American man who died in police custody. After community upheaval, arrest rates plummeted, either because of decreased willingness of police to do their jobs, decreased community cooperation with police, or both.

In some cities, experts speculate that policy changes might be having an effect. Curtailing “stop and frisk” policing in New York, for example, might have contributed to a fall in the number of illegal guns that police managed to take off the street last year. Reducing punishments for drug possession and theft in California — a reform that led to the release of nearly 4,000 prisoners — might be increasing non-homicide crime in Los Angeles. Nationally, a growing reluctance to imprison people could be overwhelming alternatives to incarceration, such as community supervision.

District Police Chief Cathy L. Lanier says that the city has been flooded with guns from states with lax laws; she points to increasing use of high-capacity magazines as one factor driving up the death toll. Ms. Lanier has also noted that increasing use of lightly regulated synthetic marijuana may be sparking some crime.

Even without full understanding of causes, certain policies make obvious sense. The lack of effective gun regulation in the United States is a disgrace that Congress must change. Cities and states need to get a better handle on spottily regulated but dangerous drugs. Given time, the introduction of crime-predicting software and better efforts to build relationships in high-crime communities might help keep a lid on violence.

If violence continues to increase, inevitably criminal justice reform will be challenged. But the bipartisan reaction against harsh incarceration gained steam for sound reasons. It shouldn’t be abandoned in mid-course; it should proceed with due care. De-incarceration shouldn’t make it difficult to lock up dangerous people, and discredited strategies, like over-intrusive stop-and-frisk campaigns, must be replaced by new strategies to confiscate illegal weapons.