A year after the onslaught of dozens of twisters killed at least 250 people in Alabama and more elsewhere in the South, federal researchers are completing a study of who died and where they were when it happened. Among the conclusions so far: Nearly half of the people who died had been advised to take shelter. Indeed, most of them did.

But many of the tornadoes were so fierce that few structures were able to withstand them.

Unlike in other tornado outbreaks, the largest group of people who died were in single-family houses -- not mobile homes -- the CDC analysis found.