A new report from the FBI comes with a troubling finding: In the first six months of 2015, the total number of violent crimes rose across the country by 1.7 percent and murders by 6.2 percent compared to the first six months of 2014.

The report follows a year of similar reports warning of a rise in murders. A Brennan Center for Justice report in December 2015 found that among 25 of the 30 most populous US cities, the murder rate was projected to increase by an average of 14.6 percent in all of 2015 compared to 2014. Several media outlets reported similar figures, including the New York Times, CNN, USA Today, and NPR.

The figures are startling because they could indicate a reversal in the US's decades-long crime drop.

But there's good reason to hold off on the panic. According to the FBI report, violent crime and murder rates are still near historic lows. The Brennan report in particular noted, "Since murder rates vary widely from year to year, one year's increase is not evidence of a coming wave of violent crime." So it's possible that the increase — although alarming — may not represent a reversal of the long-term trend toward less violent crime.

As the numbers came in throughout 2015, criminologists told me a similar story: They cautioned not to make too much of one year's figures for homicides or violent crime. The increases could be statistical blips, and some cities aren't seeing any spikes in homicides or violent crime. And if there is an increase, the theories that media outlets reported — too many guns and the "Ferguson effect" — most likely aren't the full or right explanations.

So what's really going on? We most likely won't have hard answers for years to come — the FBI won't even release its nationwide crime report for all of 2015 until late 2016, making a comprehensive look difficult. But there are some possible explanations.

Theory: The "Ferguson effect"

The most controversial — and least supported, among the criminologists I talked to — explanation is that homicides and violent crime are up because of the "Ferguson effect": The criticisms of police use of force since the shooting of Michael Brown in Ferguson, Missouri, have made cops nervous about doing their jobs, because they can now be criticized for what they think of as routine or proactive police work. At the same time, the criticisms have emboldened criminals, who now know that police are acting cautiously to avoid triggering another Black Lives Matter protest in their cities.

Proponents of the Ferguson effect theory, such as conservative Heather Mac Donald, point to arrest and shooting numbers from various cities. Take, for instance, Baltimore: After Freddie Gray died while in police custody and protests and riots ensued, the number of arrests in the city dropped, while the number of shootings and homicides increased. Similar numbers exist for other cities, including St. Louis, near Ferguson.

So perhaps arrests dropped, according to this view, because police were scared to do their jobs as a result of criticisms, and criminals took advantage of the situation, leading to more shootings and murders.

But some criminologists say these claims are highly questionable — both empirically and logically.

For one, a 2015 report by the Sentencing Project found that St. Louis's murder spike began before Michael Brown died in August 2014, which is what gave rise to nationwide protests in the first place. And statistics from Baltimore show that arrests were down and homicides and shootings were up before Freddie Gray died.

"The people who point to the Ferguson effect as the reason for the increase haven't been consulting calendars very carefully," Franklin Zimring, a criminologist at the University of California Berkeley, said in 2015.

There are other factors that may explain the drop in arrests. Baltimore, for example, had been dealing with reduced staffing levels and scheduling problems with its police force. Justin George reported for the Baltimore Sun in June 2015:

Beginning in January, patrol officers went from five eight-hour work days to four 10-hour days. The shift was part of a new union contract approved last year, and it coincided with giving officers a 13 percent raise across the board to better compete with other state agencies. To pay for the positions, 212 vacant officer positions were eliminated. … He said staffing numbers have been hurt by 367 vacancies that include injured and suspended officers. Police are working with federal partners on drug and weapon investigations. Some officers have doubled up in cars — unusual for Baltimore police — to help deal with the cellphone-holding crowds taking video of officers during routine calls.

Claims of the Ferguson effect also originated from a questionable source: police. It was St. Louis Police Chief Sam Dotson who originally labeled the phenomenon, leading Mac Donald to give it wider publicity through an op-ed in the Wall Street Journal. But there are reasons to doubt Dotson's — and other cops' — claims that this is a cause.

Media reports often treat police as impartial subject experts on crime, but it's in cops' self-interest to fault something like the Ferguson effect for a crime increase. It suggests cops aren't to blame — they want to do their jobs, but critics of police use of force are making it very difficult. It also justifies ignoring or disavowing the criticisms police have faced over the past year — if Black Lives Matter rhetoric is causing a spike in crime and murders, then maybe it's better to not criticize police and just let cops do their jobs. And it's in the interest of police to play up any increases in murders and crime to argue they need more resources and greater leeway in pursuing tougher tactics.

But criminologists repeatedly told me there's no solid evidence behind the Ferguson effect, and there are some theories to the contrary.

Theory: The opposite of the "Ferguson effect"

Charis Kubrin, a criminologist at the University of California Irvine, told me in 2015 that one cause of the murder increase might be what she characterized as the opposite of the Ferguson effect: After hearing about police brutality and perhaps experiencing it firsthand, some local communities are now uncomfortable turning to the cops when problems arise, and that in turn makes it harder for cops to stop crime.

"People don't feel like they can go to the police even if they've witnessed crimes, because they don't trust the police, and there's antagonism there," Kubrin suggested. "And police can't do the job on their own — they need the community to help them."

If true, police will need to work on repairing relationships with communities — even if they believe some of the criticisms are unfounded. Only through a rebuilt sense of trust would the public be more willing to look to law enforcement for help to settle disputes without any violence. And calling the criticisms unwarranted could stoke further distrust, since it might make communities feel unheard.

"It sends the message that you have to handle conflict on your own"

Research shows that police have a harder time getting their jobs done when they don't have community cooperation. One study published in the National Institute of Justice Journal found murders are a lot more likely to be solved when police officers are faster at securing a scene, notifying homicide detectives, and identifying witnesses. All of these tasks are easier if locals are willing to cooperate.

If police can't get that cooperation, they can't solve as many homicides, and potential repeat offenders won't be incarcerated before they carry out another shooting. And if those offenders begin to think they can get away with it, perhaps their peers will as well. "That sends a message that criminals can get away with things," Kubrin said. "And it sends the message that you have to handle conflict on your own."

Still, the idea runs into many of the same problems as the Ferguson effect — notably that the rise in murders in some cities began before Black Lives Matter protests led to heightened skepticism of the police. It's also possible that, even if this idea or the Ferguson effect are real, they won't lead to a long-term rise in the crime rate.

Theory: The rise in murders is a statistical blip or temporary uptick

It isn't the most exciting idea, but a lot of criminologists are quick to note that the rise in murders and violent crimes could be a temporary blip in the data. It's possible that the number of homicides will go up in 2015 but then continue the long-term trend down. After all, most US cities are still far below the violent crime and murder rates of the 1990s, and they're bound to go up sometime.

The FBI data shows, for instance, that there was an uptick in violent crimes in the first six months of 2012 compared to the first six months of 2011 — and then crime dropped over the next couple years. Violent crime also slightly rose in 2005 and 2006, then continued its long-term decline to a record low in 2014. It's possible a similar scenario could play out again in 2015.

Homicide data is also extremely noisy from year to year. In a city where there are only a hundred or so homicides annually, it takes just a dozen or so murders to cause what looks like a statistically significant difference. "If the number of crimes in a city goes from 100 to 115, percentage-wise that's a huge spike," John Roman, a senior fellow at the Justice Policy Center at the Urban Institute, said in 2015. "But if you compare it to 1991 when it was 480, it's a blip."

It's possible, then, that many of the increases are just a blip: Some temporary, local events caused a few more murders than there would be otherwise, and since even a small rise in murders can cause what looks like a statistically significant increase, we now have what seems like an alarming trend in some cities.

In a post in September 2015 for the Marshall Project, Bruce Frederick, a senior research fellow at the Vera Institute of Justice, cited Chicago as an example of this happening: Homicides are down in the Windy City compared with decades ago, but the trend hasn't been totally smooth from year to year, producing upticks here and there.

So it's not a sure thing that this national trend will actually hold out in the next few years. Even a temporary increase in violent crime is of course worrying, but it might not mean a reversal of the long-term trend down.

"As criminologists, we want to look at much longer time trends," Kubrin of UC Irvine said in September. "If these blips turned into month after month and year after year of increases, that's when criminologists get serious and start saying, 'Okay, what's going on here?'"

Theory: The nationwide averages obscure more mixed local trends

It's possible the cities seeing upticks aren't part of a fully nationwide trend. The Brennan Center for Justice report, for instance, found the story is more mixed: Although 18 of the 25 major US cities analyzed saw an increase in murders, seven did not.

Some cities also have much more complicated stories than their murder numbers might suggest. For example, although the Brennan Center projected a 62.8 percent increase in murders in Baltimore between 2014 and 2015, it projected a much lower 4.6 percent increase in the city's overall crime rate. New York City, meanwhile, is expected to see a 2.9 percent decline in its crime rate in 2015, even though its murder total is projected to end 6.8 percent higher than the year before.

"What's going on in the United States in 2015 is much more typical historically than the two periods where all the arrows pointed in the same direction," Zimring, the UC Berkeley criminologist, said in 2015, referring to the periods in which crime spiked in the 1960s to early 1990s and crime dropped after. "This is a very big country with lots of different regions and lots of urban structures and lots of suburban structures."

It could be that some cities are experiencing a set of unrelated local issues, leading to what looks like a nationwide violent crime trend but is in fact a series of local events that are coincidentally happening at once. Maybe Baltimore really was set on a dangerous path after the protests and riots earlier in 2015. Perhaps some cities have seen more petty, local gang disputes escalate into violence. And other cities could be seeing a rash of domestic disputes this year that turned deadly. All of these events are local, but they can look like part of a nationwide trend when they happen all at once across multiple cities.

The only way to know for certain if there is a national trend is to wait for the FBI's full 2015 report with the nationwide crime numbers, which would give some insight into what happened in each city. But for now, the Brennan Center's report is the best source — and it presents a more mixed picture that warrants some alarm but also cautions against sweeping conclusions.

Theory: Some sort of disruption caused an increase in gang violence and turf wars

One of the potential local explanations for the homicide increase is gangs.

Gang violence is always prone to disruptions. Maybe there's a shift in the drug market that drives a lot of new activity. Maybe the shooting of one gang's member causes a cascade of retaliations. Maybe the arrest of a powerful gang leader leads subordinates to an internal violent struggle over who gets to take the top job.

For example, during the Baltimore riots this year, looters reportedly stole prescription drug stocks from more than 30 pharmacies and clinics, and law enforcement officials said these drugs then flooded the streets. This kind of disruption in the local market could have caused local drug dealers and gangs to fight over who gets the turf and supply. And with that fighting came more shootings and deaths.

These disruptions tend to be local — sometimes down to the neighborhood level. So they likely wouldn't explain a nationwide violent crime or murder increase. But it does help explain the extraordinary numbers coming out of Baltimore, which is reporting a much higher spike in murders than most other cities. And it's possible similar scenarios are playing out in the other cities with violent crime increases, which could explain some of the variation from place to place as well.

Theory: The economy is improving, so more people are out and about and exposed to crime

Another, more nationally focused explanation for crime: the economy.

Over the past year, the economy continued to improve. With the improved economy, the public became more confident in their spending — and more people went out shopping, ate at restaurants, and took part in other paid public activities. But with that extra time in public, people may also have exposed themselves to more violent crime, Roman of the Urban Institute argued in September.

This is essentially a version of the "routine activities theory": When potential criminals and people are likelier to meet in public without the interference of the police, crime will likely increase.

Some evidence suggests that a better economy drives crime down. After all, crime looks like a much less lucrative option if someone can get a job instead.

But Roman argued that this isn't necessarily true in cases when the economy improves but leaves behind an underclass of Americans that is still predisposed to crime. The Brennan report suggested that the cities that experienced big murder increases in 2015 met this criteria, with high levels of socioeconomic inequality and poverty compared to other cities.

"If the average American is wealthier, there's less crime in this country," Roman said. "But crime is a function of dense clusters of unskilled young men. The question on the table is if there are fewer clusters of unskilled young men than there were a year ago. And the answer, from what I've seen, is no, there's about the same number."

He added, "The follow-up to that is if there are more people out and about who would be suitable targets to those dense clusters of unskilled young men. And the answer to that is yes — the economy is better, people are more confident, they're out more."

Theory: There are more guns

One theory propagated in media reports: More guns are driving more homicides. "The police superintendent in Chicago, Garry McCarthy, said he thought an abundance of guns was a major factor in his city's homicide spike," Monica Davey and Mitch Smith reported in the New York Times in August 2015.

The research clearly shows that places with more guns tend to have more gun-related deaths. So the claim that an abundance of guns is causing more deaths than there would be otherwise is logically sound.

But there's no evidence that there are more guns out there causing more homicides in comparison to years before. In fact, the latest numbers suggest that gun ownership is, if anything, trending down nationwide.

"The prevalence of guns is an important factor," Alfred Blumstein, a criminologist at Carnegie Mellon University, said in 2015. "But they were as available in 2013 and 2014 as they are today."

It's possible some cities' local numbers look different from the national trend. Maybe Chicago had a new influx of guns in the past year that drove an increase in shootings and homicides. But that kind of local data isn't currently available.

So more guns and access to them do tend to cause more violence and death. But there's no evidence to indicate that a new wave of guns is contributing to a new wave of crime.

As with other explanations, then, it's too early to say. It's likely that criminologists will look at cities' homicide increases for years before finding hard answers. Or, perhaps, this year's increases will turn out to be nothing more than a series of temporary upticks, and the entire debate and controversy surrounding the rise in violent crime — and Ferguson effect — will look like an overreaction to a brief statistical anomaly as the long-term trend toward less crime continues.