American life expectancy is in decline for the first time since 1993, when H.I.V.-related deaths were at their peak. But this time, researchers can’t identify a single problem driving the drop, and are instead pointing to a number of factors, from heart disease to suicides, that have caused a greater number of deaths.

A study on mortality rates released on Thursday by the National Center for Health Statistics showed that Americans could expect to live for 78.8 years in 2015, a decrease of 0.1 from the year before. The overall death rate increased 1.2 percent — that’s about 86,212 more deaths than those recorded in 2014.

Dr. Peter Muennig, a professor of health policy and management at Columbia University’s Mailman School of Public Health, said in an interview that the decline was a “uniquely American phenomenon” in comparison with other developed countries, like Japan or Sweden.

“A 0.1 decrease is huge,” Dr. Muennig said. “Life expectancy increases, and that’s very consistent and predictable, so to see it decrease, that’s very alarming.”