An unexpected increase in the United States' national death rate in 2015 might be a sign something's wrong with healthcare or culture but experts say it's still too early to make this conclusion.

The death rate rose to 729.5 deaths per 100,000 people in 2015 compared to 723.2 in 2014, according to the National Center for Health Statistics. It was one of the few times over the past 25 years the U.S. death rate has increased.

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The increase was the first such rise in a decade. Initial federal data show the rise was caused by more people dying from suicide, drug overdoses and Alzheimer's disease. What's puzzling is that a rise in the death rate among the entire U.S. population is quite rare.

"We are not accustomed to seeing death rates increase on a national scale," said Andrew Fenelon, a researcher at the U.S. Centers for Disease Control and Prevention (CDC).

"We've seen increases in mortality for some groups, but it is quite rare to see it for the whole population."

A likely culprit that might be the increase in suicides among middle-age white Americans but experts caution it's too early to conclude this statistic was a significant factor. Experts, however, said the increase was real and could signal a problem in the health of the nation if it continues.

Suicide deaths rose from 2014 to 2015, extending a troubling upward trend. The suicide rate in the U.S. shot up by 24 percent from 1999 and 2014, according to CDC.

"It's an uptick in mortality and that doesn't usually happen, so it's significant," said Robert Anderson, the chief of mortality statistics at the National Center for Health Statistics.

"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."

Tagsnational death rate, United States, Centers for Disease Control and Prevention