The U.S. death rate rose slightly last year -- the first increase in a decade, health officials reported Wednesday.

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Researchers think the increase is due to a combination of factors. The death rate from heart disease -- the nation's leading killer -- leveled off, instead of dropping the way it usually does. Meanwhile, deaths rates for accidental injuries, stroke, and some other causes increased.

The Centers for Disease Control and Prevention posted the numbers Wednesday. They are based on a preliminary look at 2015 death certificates.

The overall death rate rose to nearly 730 deaths per 100,000 people last year, from about 723 the year before.

The rate has occasionally increased, the last time in 2005. It also increased in 1999, 1993 and 1998.

It's too early to say whether 2015 will be another one-year blip or the start of a more lasting trend, said Farida Ahmad, a CDC researcher.

A lot may hinge on the heart disease death rate. In the past, declines in deaths from heart disease has offset increases in other causes, she said.

"If the heart disease rate isn't falling, there isn't a buffer," she said.

Ahmad stressed that the 2015 data is preliminary. But there is enough to reach a conclusion about a rise in the overall death rate. "We expect this to hold," she said.

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