When the rain began to pour over Green Mountain, N.C., in late September, Alison Wisely kept a close eye on the puddles growing slowly outside her window.

Hurricane Helene was churning across the American South, and Ms. Wisely, 42, and her fiancé, Knox Petrucci, 41, were hunkering at home with her two young sons. The house was hundreds of miles from any coastline.

On the morning of Sept. 27, a nearby river overflowed, and catastrophe came quickly. Floodwaters rushed toward the couple and the children — Felix, 9, and Lucas, 7. In a frantic effort to escape, all four lost their lives.

Their deaths represent only a small fraction of Helene’s terrible toll. The storm has killed more than 200 people, making it the deadliest tropical cyclone to strike the mainland United States since 2005.

Over the last three months, The New York Times collected data on Helene’s deaths from county coroners and state officials, and interviewed family members.

The findings revealed that rain, which led to flooding and landslides, was the most deadly part of the storm, followed by wind, which toppled trees. And they showed that most of the deaths happened in counties where the risk of hurricane fatalities had been considered low, according to data from the Federal Emergency Management Agency.