- Increase in violent crime worries some cities, but crime is trending downward in others
- The debate about why violent crime is up or down often becomes political
(CNN)After decades of a downward trend in crime, residents in some large U.S. cities wonder if a reversal is coming.
If you live in Baltimore, you know that May, with 43 homicides, was the deadliest month since 1972. Or if you are a Houstonian, you've probably heard that murders were up 45% through April compared to the same period in 2014.
The latest statistics in Milwaukee show a 103% spike in murders year-to-date compared with a year ago. In Atlanta, 41 people were killed in the first five months of this year compared with 27 in the same period last year, an increase of 52%.
The spike in killings in these major cities would be troubling in itself at any time, but it is especially troubling now, when policing practices, race and social policies are regularly in the news.
The video of a gunman brazenly opening fire on another man in the Bronx in May, or another gunman caught on camera firing across the street at someone in Harlem in April, spread so swiftly online that it is fair to ask if a crime wave is on the horizon.
A review of murder statistics in major U.S. cities so far this year shows an unclear picture.
While Baltimore and Houston appear to be experiencing a crime wave, comparable cities like Dallas and Los Angeles are trending in the opposite direction.
In short, it is too early to draw conclusions of a shift in the trend for violent crime.
How telling is Baltimore's deadly month of May?
Of the 119 homicides recorded in Baltimore this year, more than one-third happened in May.
As the Baltimore Sun put it in an editorial, "We don't think it is at all unreasonable to start asking questions about leadership in a city that, over the last month, was less safe by some measures than it has been at any point in recorded history."
On Wednesday, Baltimore Police Commissioner Anthony Batts said his office is asking for more federal resources -- prosecutors and law enforcement officers -- to boost the city's response to the recent uptick in crime.
Speaking at an event remembering a toddler who was killed by a stray bullet last year, Baltimore Mayor Stephanie Rawlings-Blake said last month that it is a "very, very painful time in our city."
On the other extreme is Los Angeles. Because of its large population, the city notches one of the nation's highest numbers of murders, but the trend has been shrinking violent crime.
CNN requested murder statistics for 2015 from a number of large U.S. cities. Some departments cooperated right away, while others asked for more time or formal open records requests. Among the departments that released statistics, the numbers reflected different periods. Some cities had murder statistics through May, others just through April.
For the cities where crime does appear to be trending upward, how can one know if it is a blip or a historic reversal?
"It's a little bit like the stock market. These statistics go up and down," said Harold Pollack, co-director of the University of Chicago Crime Lab. "It's like asking why did the stock market go up 75 points today."
But numbers have the power to sway, and many of these figures are being used already to bolster arguments for stronger police enforcement or a reformed police presence.
Explaining the downward trend
As policing has changed over the years, the question of what the nationwide decreases in violent crime means has been debated.
There is general agreement that larger police departments -- and more officers in the streets -- has had a positive effect on lowering crime, Pollack said.
The quality of policing has also improved over the past 20 years and the departments are better managed, he said.
Other factors are harder to quantify.
The end of the crack epidemic is believed to have contributed to the decrease in violent crime, as have other reasons ranging from the legalization of abortion to changes in the illegal drug market.
This year "may not be shaping up to be a terrific year in many cities, and it may be part of a larger pattern, but we really don't know that," Pollack said.
So what's the debate right now?
One obvious difference between last year and this year is the tensions between police officers and certain communities.
The high-profile instances of police officers killing unarmed black men stirred outrage and protests.
There is an understanding that somehow things have changed -- or must change -- in a post-Michael Brown, post-Freddie Gray, post-Eric Garner America.
The debate on whether police reform is needed or whether more aggressive policing is necessary is often political. The early 2015 murder statistics are providing evidence for both sides.
"If there's a national mood that starts to see police as the bad guys, the police as the enemy responsible for these problems, it makes it a hell of a lot harder to police," said Peter Moskos, a former Baltimore police officer and professor of policing. "One way that cops deal with that is that they just stop policing those people."
A former New York Police Department officer, Bill Stanton, agreed that an uptick in crime can be linked to police being less assertive.
"When you take away police pride and you take away giving them the benefit of the doubt ... and you're going to call them racist and you're going to prosecute them for doing nothing wrong," Stanton said, "then what happens is they're going to roll back. They're not going to go that extra mile."
CNN Political Analyst Van Jones said tying the protests over the deaths of unarmed black men to increases in crime is disingenuous.
"Police unions are trying to link any crime to First Amendment protests and cherry-picking data," he told CNN's Erin Burnett.
"This is all part of an attempt to tell black people that if we exercise our First Amendment rights, we are somehow now responsible for people who engage in crime," he said. "Why should the black community have to choose between police abuse and police neglect? That's a false choice."
The bottom line, statisticians say, is that there is not enough data to conclude if a new crime wave is upon us, or if there is, what factors are behind it.
Pollack suggested that looking at the available data through a political lens can distract from a focus on the fundamentals.
Nearly without exception, the protests over the killings of unarmed black men have been examples of police misconduct or mistakes, Pollack said.
All communities need and want good policing, and the focus should be on factors that are known to have lowered crime, he said.
Things like community policing and addressing other social issues in the communities have worked, he said.
"Public safety is a joint product of the police and the community, and each side has to trust each other, and when that trust breaks down, it's very hard for police to do its job and for the community to do do its part as well," he said.
But with the current political climate, don't be surprised if crime statistics become part of the discussion on race and policing.
"The premise of the Black Lives Matter movement, that the police are the biggest threat facing young black males today, is simply false, and the animosity that's directed to police on the streets today is having an effect," Heather Mac Donald of the Manhattan Institute told CNN's Chris Cuomo. "I've heard from many officers that they are reluctant to engage in actions that could be misinterpreted on cellphone cameras."
The accusation from the other side is that there is an intentional effort to undermine the political support black protesters have garnered.
"Conflation of the protests with a rise in crime and criminality itself kind of defames what the protests are about," New York Times columnist Charles Blow told Cuomo.