A baseball hat rests next to blood where a a man was fatally shot in the head in Chicago last month. (Joshua Lott/Getty Images)

As a number of major American cities are reporting more homicides this year than last year, the overall number of killings in the country’s biggest cities has increased in 2016 — with just one city accounting for nearly half of the total spike, according to an analysis released Monday.

Chicago has experienced almost half of the overall increase in homicides reported by major cities, a year after it was one of three cities — along with Washington and Baltimore — that accounted for more than half of the total increase in such areas, according to the analysis from the Brennan Center for Justice.

As it did last year, the Brennan Center, a law and policy institute, expressed notes of caution regarding reading too much into the increase in homicides, which have drawn national media attention and cropped up as an issue in the presidential campaign.

“Nationally, crime remains at an all-time low,” wrote the authors of the Brennan Center’s new report, which collected numbers through the first half of the year from police departments. The Washington Post reviewed a copy of the report before it was released Monday morning.

The report’s authors said their findings undercut suggestions of “a new nationwide crime wave” and instead suggest that the individual increases stem from “as-of-yet unidentified local factors” rather than any national cause.

Ultimately, they conclude that “there is not a nationwide crime wave, or rising violence across American cities,” adding: “Warnings of a coming crime wave may be provocative, but they are not supported by the evidence.”

However, the report states that the increases highlight “a serious problem” in individual cities such as Chicago, which has had a significant surge in gun violence this year. Police there said that by the end of Labor Day, the city had recorded 488 homicides, higher than the 481 killings recorded in all of last year. The Brennan Center report calls Chicago an outlier, noting that “no other large city is expected to see a comparable increase in violence.”

Chicago’s violence has been regularly cited by Donald Trump, the Republican presidential nominee, as he points to bloodshed that he says he will be able to stop if elected. In his acceptance speech at the Republican National Convention, Trump specifically pointed to the increase in killings in Chicago, Washington and Baltimore.

The Brennan Center said in a report earlier this year that those three cities alone experienced most of the increase in killings last year. However, its new analysis notes that although Chicago’s homicide numbers have continued to climb — with more killings there in 2016 than in New York and Los Angeles combined — both Baltimore and Washington have moved in the opposite direction, with fewer killings this year than last.

According to police data, Baltimore had 214 homicides through earlier this month, down from 228 at the same point last year; in Washington, there were 99 killings by Friday, down from 111 on the same date in 2015.

This new report comes after the Major Cities Chiefs Association, a group of law enforcement leaders, released data from more than two dozen police agencies showing higher homicide totals through the first six months of the year. Most of the same cities that had higher homicide totals through the first half of 2016 also had their homicide counts go up through all of last year as well.

Even as we examine crime statistics this year, a more complete picture has yet to emerge of the crime rates last year. The FBI said that the number of violent crimes nationwide was slightly up in the first half of 2015, but it has not released its full numbers for the entire year. (The bureau is expected to release them in the near future, which means that even as attention has shifted to crime numbers in 2016, the year will be largely over before we know all of last year’s figures.)

Still, experts have urged against drawing too broad a conclusion from the numbers, pointing out that rates of violent crime, including killings, are well below what they were just a quarter-century ago. And nearly half the departments that gave the Major Chiefs Cities Association homicide numbers reported fewer killings this year than last; the cities that responded to the group were roughly split between those with increases and decreases.

“There’s a lot of noise in this data,” Richard Berk, a professor of statistics and criminology at the University of Pennsylvania in Philadelphia, told The Post this summer. “We don’t understand why in any kind of systematic way.”

But some high-profile voices have said that despite the historic numbers, they remain unsettled by the figures in some places. FBI Director James B. Comey — who has publicly wondered whether increased scrutiny of police officers may be playing a role in the homicide increases — said in May that he was “very worried” about the spikes.

“People say to me, ‘Well, the increases are off historic lows,’” Comey said. “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.”

Police officials in cities that have had more homicides this year and criminologists have offered various possible explanations for the increases, including more gang violence and issues stemming from drug deals.

The authors of this new Brennan Center report said that they were not able to draw any conclusions about the causes “due to lack of data,” but they wrote that cities with issues like racial segregation, unemployment and poverty “are more prone to short-term spikes in crime.”

Comey has raised the question of whether these spikes may be related to police becoming less aggressive in an era of increased scrutiny on officers. This idea is generally known as the “Ferguson effect” and takes its name from the Missouri city where protests erupted after a white police officer fatally shot a black 18-year-old in 2014. (William Bratton, the former New York police commissioner, instead referred to a “YouTube effect” because of video recordings of police actions that can spread online.)

In a report released this summer, criminologist Richard Rosenfeld questioned whether a loss of trust in police may have spurred residents to become less likely to share information about crimes with law enforcement. Although Rosenfeld said he doesn’t buy into any specific explanation, he now believes there is a connection between crime spikes and the criticism of police.

Still, the new Brennan Center report notes that although protests spurred by high-profile videos and police uses of force “could explain rising violence in some cities … crime has not risen or fallen uniformly in the cities most affected by this trend.” Although Chicago had both high-profile police shootings and a spike in killings, the authors point to Baltimore, where killings rose last year in a city rocked by the protests over Freddie Gray’s death but did not continue to increase this year.

The Brennan Center report projects that the overall crime rate in the country’s 30 biggest cities will remain largely the same this year as in 2015.

Further reading:

What Trump says about crime in America and what is really going on

Chicago police to Donald Trump: ‘If you have a magic bullet to stop the violence,’ let us know

Trump, in answer to police chiefs, says there is ‘no noticeable partnership’ between feds and local police