When homicides spiked in Milwaukee and other cities two years ago, no one had a definitive answer for why.

Some pointed to tensions between African-American communities and police as a contributing factor after high-profile police uses of force.

But a recent federal study points to another, larger factor: the opioid crisis.

Not only did local police departments report more drug-related homicides to the FBI in 2015 and 2016, but the number of white homicide victims and suspects rose, said Richard Rosenfeld, a criminology professor at the University of Missouri-St. Louis and one of the study's authors.

“That’s notable because the dominant narrative has focused on tensions between the black community and police, and that doesn’t easily fit in that narrative,” he said.

Milwaukee mirrors that national trend.

DATABASE:Milwaukee Homicide Victims

In 2016, whites represented 12% of all homicide victims — compared to only 6% of victims the year before, according to a report from the Milwaukee Homicide Review Commission.

Preliminary data shows whites represent about 9% of homicide victims this year.

Drug-related homicides, including drug-related robbery, also spiked dramatically from 13 in 2014 to 25 in 2015, representing about 17% of all homicides. The number and percentage declined in 2016, however.

“This really didn’t come out of nowhere for us,” said Mallory O’Brien, who leads the city’s Homicide Review Commission. “I think we should be giving more consideration to the drug piece."

How drugs lead to violence

At the close of 2015, when Milwaukee had recorded a decades-high death toll of 146 homicides, authorities seemed to downplay the role of drugs in the spike and focused on other causes.

Milwaukee’s leading factor in homicide was, and remains, arguments and fights, according to police data.

But two years later, evidence is mounting that the drug trade — specifically a rising demand for opioids — was a bigger factor than first believed.

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By 2015, the turf battles over corners and houses, fought by structured gangs, had been replaced by rolling drug houses and crews, loose groups of friends or acquaintances with no formal affiliations.

A dealer’s cellphone became gold, keeping the flow of customers, drugs and money flowing. Dealers drove to their customers, often using rented or stolen cars with tinted windows and exploiting Milwaukee’s restrictive pursuit policy that allowed officers to chase if the car, or its occupants, had been linked to a violent crime. The policy was changed this fall so officers can chase cars with ties to drug dealing or reckless driving.

The Milwaukee Police Department touts its use of data and routinely uses crime statistics to identify violent crime “hot spots.” Police officials here have frequently published maps showing those areas also correlate with high rates of poverty, low educational attainment, foreclosures and other societal ills.

Those deep systemic problems are the roots of crime — but those factors have been stubbornly persistent and did not greatly change from 2014 to 2015.

So researchers and crime analysts looked at what did.

The Milwaukee Police Department created heat maps overlaying violent crime and drug overdoses and found both concentrated in the same areas.

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“Anytime you have high-value commodities being sold, any type of drugs, there’s going to be some nexus of violence associated with it,” said Milwaukee Police Lt. James MacGillis of the High-Intensity Drug Trafficking Area task force.

“They’re protecting their assets, they’re protecting their market,” he said. “Anecdotally you can say that, but then the proof is tying the two together.”

Drug-related robbery also drives violence, with assailants looking to steal drugs, money or both.

“It is not surprising that the drug economy contributes to violent crime,” Milwaukee Health Commissioner Bevan Baker said.

"Concentrated violence never operates in a vacuum,” he said. “It often co-exists in environments with other risk factors.”

Declining police legitimacy

Two years ago, much of the local and national discussion on the homicide spike focused on the so-called “Ferguson effect.”

The theory has two interpretations: The first is that a series of high-profile police shootings nationwide and protests resulted in de-policing, meaning officers backed off and made fewer traffic stops and arrests.

The second is that those same incidents further eroded police legitimacy in minority neighborhoods, making people less likely to trust the criminal justice system and cooperate with it.

“I don’t have much evidence as yet for police withdrawal,” Rosenfeld said, adding arrests rates, one measure of police activity, already had been declining before 2015.

“The other side, the legitimacy crisis, what can I say? It’s certainly plausible,” he said.

The problem is trying to measure a change in beliefs and value, he said.

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Milwaukee Police Chief Edward Flynn acknowledged opioids played a role in the homicide spike, but in an interview, he said he believed the Ferguson effect was a larger factor.

Asked about the two interpretations of the theory, Flynn said it “would be a mistake to try to weight one against the other.”

"That expansive notion of what the Ferguson effect means, it means the police being rigorously judged by the absolute most inappropriate metric and that's created more victimization and I think it will continue to do so,” he said.

Rosenfeld and others said more study will be needed, with a particular focus on the neighborhoods and blocks within cities where deadly violence is concentrated.

“The homicide increase in 2015 overall was the biggest percentage increase in a single year since 1968 and comes on the heels of a long-term decline in homicide rates,” Rosenfeld said.

“The increase was, and continues to be, real,” he said. “It undoubtedly has multiple sources that appear to have converged in time.”