The number of homicides increased in the first months of 2016 in more than two dozen major U.S. cities, going up in places that also saw rising violence last year, according to statistics released Friday.

The increases were small in some areas, and many big cities also had declines. But the numbers were particularly grim for a handful of places – Chicago, Los Angeles, Dallas and Las Vegas – where the number of homicides increased in the first three months of 2016 after killings and other violent crimes also went up in 2015.

“I was very worried about it last fall, and I am in many ways more worried, because the numbers are not only going up, they’re continuing to go up in most of those cities faster than they were going up last year,” FBI Director James Comey said. “Something is happening.”

Criminologists and law enforcement officials say the causes of the increases are unclear, but possible explanations include gang violence stemming from drug addictions.

Police have said people are dying after robberies and altercations while trying to get heroin. A number of police chiefs have expressed concern about repeat violent offenders.

Comey also suggested that greater scrutiny of police had possibly changed the way that officers and communities interact, an idea he voiced to much disagreement last year.

“I don’t know what the answer is, but holy cow, do we have a problem,” Comey said.

Criminologists say it is too soon to draw conclusions from these increases, and they point out that homicides and crime rates in general are still far below what they were a quarter-century ago

But Comey said that even with the big-picture numbers in mind, he remained concerned.

“Something people say to me, well, the increases are off of historic lows,” Comey said. “How does that make any of us feel any better? I mean, a whole lot more people are dying this year than last year, and last year than the year before, and I don’t know why for sure.”