There has been a 17 percent increase in homicides in America’s largest cities during the last couple of years, according to a report composed by Richard Rosenfeld and published by the United States Justice Department on Wednesday.

The last year and a half has not been indicative of the last two decades or so, when there has been a significant drop in violent crime — 71 percent from 1994 to 2013. In the first six months of 2015, violent crime was up 1.7 percent, according to the FBI. The progress made in the 21st century seems to be stalling.

Rosenfeld, a criminologist and professor at University of Missouri, St. Louis, conducted the study and suggests that one of the components could be a lack of confidence in law enforcement.

Police officers’ conflicts with young black males has become prevalent, and many people are suspicious, or downright contemptuous, of this striking trend. Thirty-four percent of black respondents in 2009 said they had “very little” confidence in the police to treat people equally, according to Pew Research Center. In 2014, forty-six percent of black participants expressed “very little” confidence in the same survey.

“When persons do not trust the police to act on their behalf and to treat them fairly and with respect, they … become more likely to take matter into their own hands,” Rosenfeld maintains. “Disputes are settled informally and often violently.”

Another potentially complimentary theory, proposed by FBI director James Comey, is that the increase in homicides could be linked to the higher levels of public scrutiny of the police due to media coverage and perhaps even body camera technology. Also known as the “viral video effect,” or the “Ferguson effect,” the theory goes that officers are deterred from speaking with people directly because how the situation transpires is uncertain, and going the extra mile might get them in legal trouble.

Comey explained: “There’s a perception that police are less likely to do the marginal additional policing that suppresses crime — the getting out of your car at 2 in the morning and saying to a group of guys, ‘Hey, what are you doing here?'”

Phillip Atiba Goff, psychologist at UCLA, believes that “a subtle shift in the perception of law enforcement, if the city is large enough, can lead to what looks like a significant increase in the body count. It happens very quickly.”

The data from the national study only encompasses major cities, but public perception polls included people from all over the country. “We don’t have a local sense of what’s going on,” said Goff.

That is why Rosenfeld is not convinced the “Ferguson effect” alone could have a large impact on the increase in homicide rate since most of the cities analyzed within the study did not have a decrease in arrest frequency.

He believes that a massive release of prisoners throughout the more recent years is more likely a stronger contributing factor. “The arrest rates of released prisoners are far greater than those of general population groups of the same age and race,” the report reads, citing a separate 2005 study. “As more released prisoners reenter the population, other things equal, crime rates should rise.”

Rosenfeld concludes “there are several empirical indicators and methods to evaluate alternative explanations of the 2015 homicide rise” and that “the heroin epidemic, reductions in incarceration, and the ‘Ferguson effect'” along with “others yet to be proposed, are not competing hypotheses so much as interacting components of a broader explanation.”

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