About 42 million Americans smoke — 15 percent of women and 21 percent of men — according to the Centers for Disease Control and Prevention. Research has shown that their death rates are two to three times higher than those of people who never smoked, and that on average they die more than a decade before nonsmokers. Smokers are more than 20 times as likely as nonsmokers to die of lung cancer. Poor people and those with less formal education are the most likely to smoke.

Mr. Carter said he was inspired to dig deeper into the causes of death in smokers after taking an initial look at data from five large health surveys being conducted by other researchers. The participants were 421,378 men and 532,651 women 55 and older, including nearly 89,000 current smokers. As expected, death rates were higher among the smokers. But diseases known to be caused by tobacco accounted for only 83 percent of the excess deaths in people who smoked.

“I thought, ‘Wow, that’s really low,’ ” Mr. Carter said. “We have this huge cohort. Let’s get into the weeds, cast a wide net and see what is killing smokers that we don’t already know.”