The horrific attack at a gay nightclub in Orlando, Fla., has captured the nation's attention, but the great majority of homicides are not due to mass shootings.

And in the last year or so, the murder rate has jumped in America's big cities.

"We are in the midst of a very abrupt, precipitous and large crime increase," says Richard Rosenfeld, a respected criminologist at the University of Missouri-St. Louis. He is the author of a study released Wednesday by the Justice Department examining reasons for the increase.

Specifically, murder is spiking — in urban areas. Rosenfeld says last year in the country's 56 biggest cities, homicides jumped 17 percent.

"That's a far larger percentage increase than in nearly any other year we've seen over the last couple of decades," Rosenfeld says.

When there's a spike in the numbers like that, criminologists look to see if it correlates with other trends — such as a surge in the illegal drug trade, or a wave of ex-convicts getting released from prison.

While both those things have happened in recent years, Rosenfeld's analysis shows the timing isn't right — they don't quite sync up with the spike in murders.

But there's another potential explanation that does, what Rosenfeld and others call the "Ferguson effect." It refers to the August 2014 shooting death of an unarmed 18-year-old African-American man by a white police officer in Ferguson, Mo.

Rosenfeld concludes in his study that some of the increase may be attributable to "some version of the Ferguson effect."

Notice, Rosenfeld says "some version." That's because people disagree about what that effect is. It could be about police holding back, afraid they'll be criticized later for what they see as doing their job. Or you could look at it from the opposite point of view: that it's a matter of citizens — especially black people — losing faith in local cops.

"When the perceived legitimacy of the police is in decline, community members take matters into their own hands," Rosenfeld explains. "Because they perceive that the police are simply not going to provide the kind of protection the community desires."

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