These deaths represented a fraction of the homicides that occurred across this country during the first half of this year, many with relatively little notice. They also occurred in places that have seen homicides increase in the first half of 2016, something authorities say has been the case in a string of cities across the country.

Police departments in many major cities and metropolitan areas say that homicides and other violent crimes are up midway through 2016 over the same point last year, reports that come as the country’s levels of bloodshed have become a recurring topic in the presidential campaign.

More than two dozen police agencies say killings in their cities were up at the midyear point, in some cases outpacing their 2015 homicide counts by dozens of deaths, according to statistics released Monday. Much of this violence is continuing in the same places that also saw violence also increase last year, as most cities with higher homicide totals for the first half of 2016 also reported more killings for all of 2015.

Experts urged caution in reviewing the numbers, saying that they do not represent a trend and noting that nearly half of the cities that released homicide numbers this week reported fewer killings this year than last year. As was the case earlier this year, cities releasing homicide figures appeared roughly split between those with increases and decreases.

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These figures come as the country’s violent crime rates are still far below what they were just a couple of decades ago, yet they have still caused worry in cities across the country. Law enforcement officials, criminologists and other experts have been unable to explain precisely why homicides have continued to increase in some places.

“It’s something that we ought to be concerned about,” said Darrel Stephens, executive director of the Major Cities Chiefs Association, the group of law enforcement leaders that collected and released this data. “The increases, even though the numbers aren’t big in most cities, there’s still an increase over what we’ve seen in years past, and there are lots of cities … seeing some kind of increase in violent crime.”

Violent crime took center stage last week during Donald Trump’s dark and foreboding acceptance speech at the Republican convention, as he delivered remarks laden with questionable assertions about a nation afflicted with “crime and terrorism and lawlessness.”

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Trump painted a dire picture of a nation is “a more dangerous environment than frankly I have ever seen,” comments that do not particularly match up with what is happening in America and the realities of a country where worries about crime often outpace actual crime levels.

President Obama, speaking the day after Trump, echoed many criminologists in pointing out that while violence has increased in some places, the crime rate nationwide remained far lower than it was a quarter-century earlier.

“As disturbing as some of the upticks in crime that we’ve seen in some of our cities around the country, including my home town of Chicago, violent crime is substantially lower today than it was 10 years ago, 20 years ago or 30 years ago,” he said.

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The numbers released Monday cover only some parts of the country, coming from a little more than four dozen police agencies. They do not include every police department that reported figures for the first three months of 2016, nor every agency that provided data for all 2015, because some have not compiled or submitted their data, said Stephens, a former police chief in Charlotte. As a result, this data provides only a glimpse, rather than a full picture, of what is happening in the country’s biggest cities.

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(The full, nationwide picture of crime in the United States this year won’t be available until the latter part of next year. The FBI still has not actually released its full data for 2015 yet, numbers that the bureau is expected to release later this year. Preliminary data released by the FBI in January for the first half of 2015 showed an uptick in murders and violent crime nationwide.)

These new numbers, for example, do not include violent crime numbers from the New York City Police Department, the country’s largest police force. According to police in New York, murders and shootings both declined through the first half of this year from a year earlier. Through June 30, there were 161 murders in New York, down from 172 at the same point a year earlier; shootings declined to 435 from 545 over the same period, police said.

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And the new numbers inevitably represent a snapshot in time, which is unavoidable when looking only at a set period. But this means that a matter of days or weeks dictate whether a city reports an increase or a decrease in homicides.

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Take Washington, where Matheno El was apparently the last homicide victim of the first half of 2016. Police in Washington reported that midyear homicides had increased to 68 from 63 at that point last year, which put the nation’s capital among those cities with an increase for the first half of 2016.

However, as of Monday, homicides in Washington had actually dropped to 70 from 79 at the same point last year. Similarly, the Philadelphia police had said that that as of the midyear mark, homicides there were up to 122 from 115. By Sunday night, the 143 homicides in that city marked a slight decrease from the 145 killings a year earlier.

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“There’s a lot of noise in this data,” said Richard Berk, a professor of statistics and criminology at the University of Pennsylvania in Philadelphia. He said that recently, his city had multiple shooting homicides in one day, then none the following day. “We don’t understand why in any kind of systematic way. You can’t make too much [of the numbers] unless there’s a really good pattern.”

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Berk said that human beings, by nature, look for patterns, comparing it to how announcers say a basketball player who sinks a series of three-pointers must be on a hot streak. He said it would be one thing if more of cities reported similar changes but noted that of the 47 cities releasing full figures for the first half of 2016, nearly as many reported decreases (21) as increases (26).

“It would be different if all the cities showed a similar increase or similar decease,” he said. “Then you could say the 50 largest cities are showing a similar effect and they’re in different parts of country. That says there might be something systematic going on. But they don’t.”

FBI Director James B. Comey, who has publicly wondered whether increased scrutiny on police officers may be playing a role in the homicide increases, said this year that he was “very worried” about the homicide increases. He expressed particular concern about killings largely grouped in certain areas away from tourists and busy business districts.

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“I worry very much it’s a problem that most of America can drive around,” Comey told reporters during a briefing. He pointed out that the surge in violence in places like Chicago or Las Vegas was not necessarily visible around the areas frequented by tourists in those areas, adding: “It’s … happening in certain parts of the cities. The people dying are almost entirely black and Latino men. We can’t drive around that problem.”

Berk said that because crime occurs in specific areas, it does not make sense to look at citywide figures.

“Crime is a neighborhood phenomenon,” he said. “You have to look at particular neighborhoods, neighborhood by neighborhood, to see what’s going on.

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“Whatever’s going on in Philadelphia isn’t universal,” Berk continued. “What’s going on in Chicago on the South Side is different than whats going on in the North and West. To talk about a city as a unit doesn’t make any criminological sense.”

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The biggest overall increase, in terms of sheer homicides, was seen in Chicago, a city that has seen a staggering surge in killings and homicides this year. There were 316 killings in the first half of this year, up from 211 last year, putting Chicago on pace to potentially top 600 homicides in a year for the first time since 2003. The city also reported 1,321 non-fatal shootings by the end of June, eclipsing the 875 shootings last year.

Eddie Johnson, the police superintendent overseeing the embattled Chicago department, said the department was committed to working with residents to help stem the violence, attributing much of the bloodshed to gang members using illegally acquired firearms.

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“While the vast majority of Chicago is a safe and growing city, we know that communities that have historically struggled with violence continue to bear the burden of gang members committing crimes with illegal guns,” he said in a statement. “We have made clear to these criminals that we know who they are and we are using every resource at our disposal to hold them accountable for their actions.”

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The Chicago police have said that they are working to fight the number of illegal guns on the city’s streets, taking possession of nearly 5,000 guns and arresting more than 1,530 people for gun-related offenses, officials said. The police department has also targeted gang members with raids and beefing up community policing.

In the data released Monday, police departments across the country reported more than 300 additional homicides combined through this year over last year. Some cities have shown relatively small increases, like Los Angeles reporting five additional homicides this year. But just a handful of cities accounted for much of the overall increase this year, including Chicago, which alone had a third of the overall uptick in homicides.

Another city that accounted for an unusually large share of the homicide increase was an obvious outlier. Orlando’s homicide tally would have risen in 2016 even without the 49 people killed in the attack at Pulse nightclub last month, but the city’s figure includes victims of that rampage, giving the city a stratospheric increase.

(Cities have had differences of opinion over how to account for events like the Orlando shooting, which authorities deemed a terror attack. The District included victims of the Navy Yard shooting in its homicide count for 2013, while Boston did not include the three people killed in the Boston Marathon bombing that same year.)

The killings in Chicago, Orlando and Las Vegas this year account for two-thirds of the total homicide increase seen in the new figures. Something similar happened last year: A Brennan Center analysis determined that more than half of the increase in homicides last year in major cities came from Washington, Chicago and Baltimore. Like Washington, Baltimore — a city that had a horrific increase in killings last year — has also reported fewer homicides as of this month, falling to 157 by mid-July from 170 at the same point in 2015.

During his remarks last week, Trump invoked Washington, Chicago and Baltimore to note their statistics from last year, without mentioning that the killings had declined in two of the places this year. Stephens, discussing that speech and others made at the Republican National Convention last week, was critical of these kinds of remarks at the event for what he called bringing up crime numbers without discussing ways to confront any violence.

“What they’re doing is using these statistics that certainly are not representing a trend in any way, by any stretch of the imagination yet, and turning them into something for political purposes, which is pretty disingenuous in my mind,” he said. “You can almost expect that kind of thing to happen, and it’s a shame that we’re really strong on the rhetoric and a little short on the action.”

Overall homicide totals — the kinds of things that get plastered on headlines around the new year and invoked during speeches by presidential candidates — can still get more attention than daily crime reports or quality-of-life issues.

“Today, every police chief knows how many homicides they’re at, whether they’re up or down in comparison to last year,” said Chuck Wexler, director of the Police Executive Research Forum. “When homicides are up in cities, mayors notice, city council members notice. … That filters down throughout the department.”

Stephens said that even if the numbers do not show a trend nationwide, police leaders across the country are still worried about what’s happening and trying to figure how to tackle it. He said police chiefs “want to have a much better sense of whats going on in communities other than their own,” adding: “They want to have a sense of whether it’s just them or whether other citizens are experiencing these kinds of problems.”

Police chiefs in different cities have offered varying explanations for the violence, Stephens said. Some mention gangs as the source of the increases. Many others mention violence related to drugs, and “everybody eventually gets to guns” when speaking about sources of the violence, he said.

Still, Stephens said that even with the numbers trailing behind decades-long trends, he still said the homicide figures his group collects should be a reason for some measure of worry.

“As a nation, we ought to be concerned about it,” he said. “Even if we had decreases in every one of our cities, you still have too many people killed in America by homicide, more than any other industrialized nation in the world, and mostly because of gunfire, and mostly in high poverty areas of our communities with a significant impact on African American populations. Yes, we ought to be concerned about it.”