Tiffany West, right, leans on Jamika Smallwood while attending a candlelight vigil for Wesley West, the victim of a homicide in Washington, D.C., last year. (Amanda Voisard for The Washington Post)

Hundreds more people were killed in major U.S. cities last year than the year before, the first substantial increase in homicides in more than two decades. Meanwhile, though, total violent crime increased just slightly, while there was no change in overall urban crime rates, posing a puzzle for criminologists and law enforcement.

An analysis from the Brennan Center for Justice in New York published this week concludes that the number of homicides in the country's 30 largest cities increased 14.5 percent last year compared to 2014. The results broadly confirm previous research by The Washington Post based on the 50 largest cities, which concluded that the number had increased by close to 17 percent.

Yet the Brennan Center also found that overall crime in those major cities was essentially unchanged after declining 0.1 percent. Violent crime had increased 3.1 percent. These figures corroborate an earlier analysis from the FBI, which issued a preliminary report based on data from the first six months of last year and also found little change. The bureau recorded a nationwide increase in violent crime of 1.7 percent during that period, along with a decrease in property crime of 4.2 percent.

The data on broad categories of crime complicate a popular theory among commentators and public figures: that the increase in the number of homicides could be the result of increased public scrutiny of law enforcement. Following the deaths of several young black men in altercations with the police, according to this theory, the risk of prosecution or negative attention from the press has made cops reluctant to do their jobs.

If police in major cities were avoiding talking with civilians, hesitating to engage with suspicious people as well as potential witnesses and informers because of the risk of something going wrong, criminologists might expect that petty crimes and street violence would increase.

Several cities -- notably Baltimore and Cleveland -- recorded alarming increases in the numbers of homicides following major controversies over police conduct, The Post found.

Denise Lu / The Washington Post

At the same time, the number of homicides increased only slightly in New York -- where relations between law enforcement and civilian leaders had been severely strained for several years even before the death of Eric Garner in a highly publicized case in 2014. Overall, lethal violence in New York has declined 35 percent over the past five years.

The data so far suggest there is no easy answer to the question of why more people were killed in American cities last year, making the problem that much more urgent for experts on law enforcement and crime.