NEW YORK — U.S. drug overdose deaths had been most common in Appalachia and other rural areas in recent years, but they are back to being more concentrated in big cities, according to a government report Friday.

The Centers for Disease Control and Prevention found that both urban and rural overdose death rates have been rising, but the urban rate shot up more dramatically after 2015.

That probably is due to a shift in the current overdose epidemic, said Dr. Daniel Ciccarone, a drug policy expert at the University of California, San Francisco.

The epidemic was initially driven by opioid pain pills, which were often as widely available in the country as in the city. But then many drug users shifted to heroin and fentanyl, and the illegal drug distribution system for those drugs is more developed in cities, Ciccarone said.

Another possible explanation: rising overdose deaths among blacks and Hispanics, including those concentrated in urban areas, he added.

“Early on, this was seen as an epidemic affecting whites more than other groups,” he said. “Increasingly, deaths in urban areas are starting to look brown and black.”

The report said the urban overdose death rate surpassed the rural rate in 2016 and 2017. Rates for last year and this year are not yet available, but experts doubt it will flip back again any time soon.

The difference between the urban and rural counties was not large. In 2017, there were 22 overdose deaths per 100,000 people living in urban areas — counties with large and small cities and their suburbs. There were 20 per 100,000 in rural areas — non-suburban counties with fewer than 50,000 residents.

Diego Cuadros, a University of Cincinnati researcher, said the findings are consistent with what he and his colleagues have seen in Ohio.

The nation is battling the deadliest drug overdose epidemic in U.S. history. About 68,000 Americans died of overdoses last year, according to preliminary government statistics reported last month.

The CDC found the urban rates are driven by deaths in men and deaths from heroin, fentanyl and cocaine.

Women still die of overdoses at higher rates in rural areas, the CDC report found. And death rates tied to methamphetamine and prescription opioid painkillers remain higher in rural areas, too.

Experts interviewed by The Associated Press were unable to immediately explain one of the report’s findings: The urban and rural death rates were nearly identical for people ages 25 to 44, but the urban rate was significantly higher in other age groups, particularly in those ages 45 to 64.