A nurse, Niels Högel, admitted to dozens of murders but might have killed as many as 300 patients. So what prevented colleagues from stopping one of the world's deadliest serial killers? Melissa Eddy of The New York Times investigates.

The new nurse arrived at the intensive-care unit of Delmenhorst hospital with a solid letter of reference, describing him as someone who worked "independently and conscientiously." In a crisis, it said, he reacted "with consideration" and was "technically correct."

It gave no indication that officials at his former hospital in Oldenburg, Germany, had grown deeply suspicious about the number of deaths