You can track annual changes in homicides rates and total homicides in American cities at AmericanViolence.org -- but the site won't tell you why rates go up or down.
If you look at the top nine cities by population, all of those with more than one million residents, you can spot that something exceptional happened in 2013: Homicide rates and total victims dropped in every one of them, as seen above in a chart generated by the site.
Altogether, there were 356 fewer homicides in the nine big cities -- down from 2,184 in 2012 to 1,828 in 2013 -- for a reduction of 16 percent.
Combining just New York City, Los Angeles, Chicago and Philadelphia, there were 305 fewer homicide victims -- for a 20 percent drop.
We do know that half of America's gun homicides in 2015 were clustered in just 127 cities and towns, even though they contain less than a quarter of the nation’s population, according to a report from the Guardian.
And that most homicides in the U.S. are committed with firearms, especially handguns, according to the National Institute of Justice.
So, what happened in 2013?
The economy continued pulling out of the Great Recession in 2013 but only very slowly, according to The New York Times.
The Bureau of Labor Statistics indicated that unemployment continued downward, but not dramatically.
So, was is the weather? The year brought the fewest tornadoes in recent memory and a largely absent Atlantic hurricane season, according to The Washington Post.
Did it have anything to do with Newtown? Twenty children were among the victims of the Sandy Hook Elementary School shooting, which shocked the nation in December, 2012. (Business insider has all their names and photos.)
"We have endured far too many of these tragedies in the last few years," including those on "countless street corners in places like Chicago and Philadelphia."
During his State of the Union address one month later, Obama discussedObama discussedObama discussed Newtown and Aurora and other mass shootings but also remembered 15-year-old Chicago shooting victim Hadiya Pendleton, who had performed at his inauguration three weeks earlier.
There are many more theories but the cause of the dramatically reduced homicide rates of 2013 remains unclear.
According to American Violence, homicide rates also went down in all of the largest cities in 2008, the only other instance so far during this century.
What do you think was making the difference when homicide rates dropped and what do you think we should do to make it happen again?
Image Source: AmericanViolence.org: About.