Even when taking into account the aging population, the death rate in the U.S. for 2015 increased for the first time in a decade, according to preliminary data from the Centers for Disease Control and Prevention (CDC).

The age-adjusted death rate jumped to nearly 730 deaths for every100,000 people, up from 723 in 2014. The increase appears to be linked to a rise in the age-adjusted death rates for Alzheimer’s (29 in 2015 versus 25 in 2014) and drug overdoses (15 in 2014 versus 14 in 2014), along with small increases in suicide, heart disease, gun-related injuries, accidental injuries, stroke and hypertension.

The number of deaths attributed to HIV and cancer decreased between 2014 and 2015.

The last increase in 2005 was attributed to a particularly bad flu season. While increases for the rate that covers the entire population are rare, experts said it’s wise to wait for next year’s data before speculating on whether the long-term downward trend in the death rate has reversed.

“It’s an uptick in mortality and that doesn’t usually happen, so it’s significant,” Robert Anderson, the chief of mortality statistics at the National Center for Health Statistics, told the New York Times. “But the question is, what does it mean? We really need more data to know. If we start looking at 2016 and we see another rise, we’ll be a lot more concerned.”

One factor is the small uptick in heart disease deaths. While the CDC said the 0.4 percent wasn’t statistically significant, it’s the first time in 22 years the rate didn’t decline. Because of the influence heart disease deaths can have on the overall rates, a small increase may have failed to offset the rise in drug overdoses and Alzheimer’s deaths as it had in past years.

While the preliminary data doesn’t break down deaths by demographics, several recent studies have suggested the overall rise could be attributed to a rise in mortality among whites.

A 2015 study published in the journal Proceedings of the National Academy of Sciences of the United States of America documented an increase in deaths confined to white non-Hispanic men and women between 1998 and 2013, which was “largely accounted for by increasing death rates from drug and alcohol poisonings, suicide and chronic liver diseases and cirrhosis.”

In a separate analysis, the Washington Post identified increases in death rates based on gender and where people live. It found the jump in mortality for whites is more pronounced in smaller cities or rural areas, and higher among women than men.