The New York Times reports that at least 35 American cities have been seeing more murders, more violent crimes, or more of both this year than last year. Some cities—Baltimore, Milwaukee, St. Louis, Washington—are undergoing unusually large leaps in the number of homicides.

Before we talk about that, some important caveats. We do not yet have national crime figures for 2015. Many cities are not seeing such surges, and in some spots the numbers are declining. In some of the places seeing increases, the spikes are small and may represent random fluctuations. If you look at New York's numbers, for example, you'll see that while murders are more common now than at this point last year, they're still lower than they were two years ago. So is 2015 a particularly bad year for New York, or was 2014 particularly good? Chicago's increase in homicides is more substantial than New York's—20 percent rather than 9 percent—but again, it comes after a year when the numbers were unusually low. (And there are reasons to doubt some of Chicago's recent low crime figures, so I'd be especially wary about drawing quick conclusions where that city is concerned.)

On top of all that, there's the simple fact that we don't know yet whether this is a blip or a trend. In 2005, after crime had been declining for more than a decade, the FBI revealed that homicides, robberies, and aggravated assaults had all grown more common in the previous year. The Police Executive Research Forum promptly produced a paper titled "A Gathering Storm—Violent Crime in America," which opened with a warning that such numbers could be "the front end of a tipping point of an epidemic of violence not seen for years." Then the drops in all three rates resumed.

But if it's premature to say there's been a nationwide reversal in America's long-falling crime rates, it's very true that several cities are facing sudden spikes in serious crimes. And while the reasons for those increases are unclear, there's been a concentrated effort among the opponents of criminal justice reform to blame them on a so-called "Ferguson effect." This notion gets mentioned in the Times piece:

Among some experts and rank-and-file officers, the notion that less aggressive policing has emboldened criminals—known as the "Ferguson effect" in some circles—is a popular theory for the uptick in violence.

That's a mild way to describe the thesis. It's often stated in much nastier terms, with an accusation that the critics of abusive police practices have blood on their hands. Most infamously, Heather Mac Donald wrote a Wall Street Journal op-ed in May that said "the intense agitation against American police departments" was the "most plausible explanation of the current surge in lawlessness." (And might police departments in turn bear any blame for inspiring such agitation? Not as far as Mac Donald's concerned.)

What's the alternative view? Well, there's We aren't even sure why crime across America was just FALLING, and that went on for more than two decades, so it's a little early to draw sweeping conclusions about the last eight months. Beyond that, given that some cities are seeing higher rates while others aren't, we may be more likely to find compelling explanations for the increases by looking at local conditions than by searching for a national pattern. The Times reports, for example, that New Orleans' increase in homicides does "not appear to reflect any increase in gang violence or robberies of strangers, but rather involved killings inside homes and cars by people who know their victims—particularly difficult crimes to predict or prevent." Whatever might explain that increase, it isn't exactly grist for the Ferguson-effect crowd: As the city's police superintendent tells the Times, that's "not a situation that can be solved by policing."

But if we're looking at local factors, we also have to consider the possibility that a Ferguson effect is at work in some locations but not in others. On the paper's list of cities where homicides are going up, the two with the highest rates are St. Louis and Baltimore, the very places where protests against police killings (of Michael Brown in the first case, Freddie Gray in the second) turned into actual riots. Could those be examples of the Ferguson effect in action? If not the sleazy version of the argument that blames reformers for crime, then at least the milder stance described by the Times, where criminals are emboldened because police are pulling back?

Not in St. Louis. As the criminologist Richard Rosenfeld has pointed out, the murder rate there started rising before Michael Brown died, then reverted to the previous year's pattern as the controversy around his death was taking off. That's hard to blame on the events in Ferguson.

With Baltimore their case is stronger, though still cloudy. Just to give you a sense of how bad Baltimore's summer has been, there have been more homicides in the city this year than in New York. That's total homicides, not homicides per capita, even though New York has more than 13 times as many people. And other crimes have headed upward in Baltimore too. Believers in the Ferguson effect can note that these numbers skyrocketed after the April riots, and that the number of arrests dropped deeply at the same time (though they've been rising again recently). The police presence around town also decreased after April. It is often suggested that these changes reflected an increased unwillingness, Ferguson-effect-style, to police aggressively.

On the other hand, the rise in the number of shootings actually started before Freddie Gray died, suggesting some deeper dynamics are at play, though the problem did intensify enormously after the riots. And before you rush to attribute the decreased police presence to post-riot wariness, you should note that the department has also been spread thin lately, with a substantial number of vacancies on the force. Nor is less policing the only possible explanation for the crime wave. With more than 30 pharmacies looted during the riots, there was an influx of new opiates onto the black market; it is widely believed that this disrupted the usual drug distribution patterns and set off violent disputes. And the Baltimore government has been in a state of even deeper dysfunction than usual this year, a fact that's had its own effects on both citizen/cop relations and police morale. So even in the city where the case for a Ferguson effect is strongest, there's a host of other factors to disentangle.

Here's one piece of good news: It's September. Many violent crimes happen more frequently in the warmer months, whether or not the annual crime rate is surging. I can think of a few cities that'll be eager to exit the long, hot summer and enter a long, cool fall.