May 19, 2016 6:13 AM EDT

With the approach of summer, a season when crime historically spikes, U.S. cities are already seeing increases in violence from 2015, when murder rates rose in many metropolitan areas after decades of decline.

Homicides are up this year in roughly 30 cities, according to data released on May 13 by the Major Cities Chiefs Association (MCCA) police group, but most of the significant upticks are concentrated in a handful of cities like Chicago, which had 141 homicides in the first three months of 2016, compared with 83 in the same period in 2015. Dallas, Jacksonville, Fla., Las Vegas, Los Angeles and Memphis all saw sizable increases too. Overall, murders increased by 9% for the 63 cities surveyed, and numbers for rape, robbery, aggravated assault and nonfatal shootings were up as well.

The reasons behind the crime spikes remain a puzzle to criminologists, who caution that they could be a temporary blip following historic lows. But FBI Director James Comey has repeatedly linked the rise to a so-called Ferguson effect, theorizing that officers have engaged in less aggressive policing over concerns about being recorded and potentially charged with a crime.

So far, though, there’s little evidence to back up that claim. “I’m not convinced that’s the case,” says Darrel Stephens, executive director of the MCCA, adding that drugs, gang violence and repeat offenders all play a role.

In fact, half of the cities surveyed saw drops in crime, including Charlotte, N.C., Houston, Oakland, Calif., Tulsa, Okla., and, most significantly, New York City, which tallied just 68 murders in the first quarter, compared with 85 in 2015, the fewest on record for the city.

This appears in the May 30, 2016 issue of TIME.

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