“I was very worried about it last fall, and I am in many ways more worried, because the numbers are not only going up, they’re continuing to go up in most of those cities faster than they were going up last year,” FBI Director James B. Comey, who got an early look at the numbers, said Wednesday. “Something is happening.”

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Criminologists and law enforcement officials, including Comey, say the causes of the increases are unclear, and they offer a variety of possible explanations, including gang violence and bloodshed stemming from drug addictions. Comey also again suggested that greater scrutiny of police had possibly changed the way officers and communities interact, which he said may be a factor in the uptick in homicides, an idea he voiced to much disagreement last year.

“I don’t know what the answer is, but holy cow, do we have a problem,” Comey said.

Criminologists say it is too soon to draw conclusions from these increases, and they point out that homicides and crime rates in general are still far below what they were a quarter-century ago, and that urban areas are much safer than they were just a few decades ago. In Chicago, which is facing a mammoth increase in shootings and other violence, the city is on pace to have more than 500 killings for just the third time since 2004; by comparison, Chicago recorded at least 800 victims of homicide each year between 1991 and 1995 before the numbers began declining.

The Brennan Center, a law and policy institute in New York, said in a recent analysis of crime occurring in big cities last year that the country is seeing “historically low levels of violence.” But Comey said that even with the big-picture numbers in mind, he remained concerned.

“Something people say to me, ‘Well, the increases are off of historic lows,'” Comey said during a discussion Wednesday with reporters at the bureau’s headquarters in Washington. “How does that make any of us feel any better? I mean, a whole lot more people are dying this year than last year, and last year than the year before, and I don’t know why for sure.”

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Law enforcement leaders in big cities that have experienced more homicides have offered varying causes, said Darrel Stephens, executive director of the Major Cities Chiefs Association, the group of law enforcement leaders that released the data.

In some places, gangs have been viewed as the primary source of violence, while police chiefs in other cities have said people are being killed in robberies and altercations while trying to get heroin, Stephens said. A number of police chiefs also have expressed concern about repeat violent offenders.

“If you put it in perspective, it’s much, much lower than what we experienced in the ’90s,” Stephens, a former police chief in Charlotte, said in an interview Friday. “But still, for me and others, the fact that we’ve had these spikes in different cities is not something that should be ignored. And I can tell you that the police are not ignoring it in places that are experiencing this.”

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Stephens cautioned that his group’s data — gathered from more than five dozen police agencies — was not nationally representative, because it focused only on the country’s biggest cities and counties.

He said the group began collecting these statistics only last year after police chiefs in Washington, Chicago and other cities expressed concerns about what they were seeing in their communities. There had not been a need to gather this data before because crime had been falling for two decades, Stephens said.

All told, half of the agencies reported increases in homicides, while the other half reported that killings remained even or below the same numbers seen last year. Some of the numbers defy easy explanation. In Los Angeles and Memphis, police reported double-digit increases in homicides, while the number of nonfatal shootings was flat or down.

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Other cities have experienced worsening violence since the three-month window covered by this data. Philadelphia numbers reported to the police group showed one fewer homicide in the first three months of this year than last but more than 75 additional nonfatal shootings. As of Friday, the number of homicides had surged 15 percent ahead of the same period last year.

Even in cities where more killings are occurring, “those homicides are not randomly distributed around the city,” said Richard Berk, a professor of statistics and criminology at the University of Pennsylvania.

“Crime, like politics, is local,” Berk said. “This stuff all occurs in neighborhoods on much more local levels. … It’s not about a city as a whole, it’s about neighborhoods.”

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The Brennan Center analysis last month found that just three cities — Baltimore, Chicago and Washington — accounted for more than half of the overall increase in homicides last year in the country’s 25 biggest cities.

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Chicago police have said that most of the increase in violence there is driven by gang members using illegal guns and that the activity is largely concentrated in a handful of areas in the city’s south and west. Chicago Police Superintendent Eddie Johnson also has pointed to what he described as a small number of previously known offenders “driving the violence” there. Cathy L. Lanier, the D.C. police chief, raised similar concerns about the District last year.

Berk cautioned against drawing sweeping conclusions about the entire country on the basis of the new data.

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“Some parts of the city are safe, always, and some parts of the city are unsafe,” Berk said. “I live in Philadelphia, there are some neighborhoods I won’t go into after dark.”

Comey said he had trouble accepting that “there isn’t something broad” playing into what’s happening, but he acknowledged that it was possible that the “jump in homicides in different cities all over the country is driven by factors in those particular areas.”

During his remarks, Comey said he was worried about this being “a problem most of America can drive around.” As examples, he said that on Chicago’s Magnificent Mile or the Las Vegas Strip, people may not be as acutely aware of the killings in other parts of those cities.

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“It’s happening in certain parts of the cities, and the people dying are almost entirely black and Latino men, and we can’t drive around that problem,” he said.

In his remarks this week, Comey revisited an idea he put forward last fall, saying that police may have become less aggressive in an era of increased scrutiny on officers. This idea is generally known as the “Ferguson effect,” taking its name from the Missouri city where protests erupted after a white police officer fatally shot a black 18-year-old in 2014. New York City Police Commissioner William Bratton has referred to a “YouTube effect” because of video recordings of police actions that can spread online.

However, pointing to high-profile videos does not explain the surge in violence in some places and a decline in others. New York City in 2014 had one of the most visible incidents caught on camera in recent years when a man in Staten Island died after being placed in a chokehold by an officer. As of last week, homicides in New York this year were down 13 percent compared with the same point last year.

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In Cincinnati, where a police officer was charged with murder after being recorded shooting a driver during a traffic stop, the number of homicides through last week had reached 24, up from 22 last year. Jacksonville, Fla., had no similar video that got nationwide attention and was played on a loop on social media and cable news, but the number of killings in that city in the first three months of 2016 had increased to 30 from 18 homicides at the same point last year.

On Wednesday, Comey said he resisted the term “Ferguson effect,” instead saying he was referring to “a sort of viral video effect” that he thought “could well be at the heart of this or could well be an important factor.”

“He is not the only one that has that perspective,” Stephens, executive director of the Major Cities Chiefs Association, said Friday. “It’s not one that I share.”

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Stephens said that although he did not agree with Comey, he admired and appreciated the FBI director for using his sizable platform to share what he was hearing from law enforcement officials.

Comey’s suggestion about the effect of greater scrutiny of policing connects with a sentiment expressed by some current and former officers, their relatives, and other former law enforcement officials, who have described feeling increasingly buffeted by criticism and said they feel like police are unfairly painted as villains.

When Comey previously made similar remarks, he drew pushback from some law enforcement officials, civil rights activists and the White House. After he said in October that the “crisis of violent crime” could be linked to “a chill wind blowing through American law enforcement,” President Obama said there was no evidence to suggest officers were pulling back, and Attorney General Loretta E. Lynch said the same.

On Friday, the White House responded to Comey’s new remarks by saying again that Obama had seen no evidence to suggest that officers were policing less aggressively because they fear being recorded.

“This administration makes policy decisions that are rooted in evidence, that are rooted in science,” Josh Earnest, the White House press secretary, said during a briefing Friday. “We can’t make broad, sweeping policy decisions or draw policy conclusions based on anecdotal evidence. That’s irresponsible and ultimately counterproductive.”

Earnest said it was a problem that some cities “are experiencing a troubling surge in violent crime,” adding: “But there’s not evidence at this point to link that surge in violent crime to the so-called viral video effect, or the Ferguson effect. There’s just no evidence to substantiate that.”

“There isn’t evidence out there to draw any firm conclusions about what’s happening,” Earnest said.

Stephens pointed out that while recordings of police have attracted much more attention recently, people have been using cellphones to record officers’ actions for as long as these devices have been able to record video. Although this isn’t necessarily a new phenomenon, the scrutiny is magnified in an era of increasing focus on how law enforcement personnel use force.

“Police, they receive a lot of negative feedback and the sense that all of these problems that we’re encountering are the fault of the police,” he said. “And they don’t feel good about it, but that doesn’t stop them from doing their work.”

Ellen Nakashima contributed to this report.