THIRTEEN years ago, researchers at the United States Holocaust Memorial Museum began the grim task of documenting all the ghettos, slave labor sites, concentration camps and killing factories that the Nazis set up throughout Europe.

What they have found so far has shocked even scholars steeped in the history of the Holocaust.

The researchers have cataloged some 42,500 Nazi ghettos and camps throughout Europe, spanning German-controlled areas from France to Russia and Germany itself, during Hitler’s reign of brutality from 1933 to 1945.

The figure is so staggering that even fellow Holocaust scholars had to make sure they had heard it correctly when the lead researchers previewed their findings at an academic forum in late January at the German Historical Institute in Washington.

“The numbers are so much higher than what we originally thought,” Hartmut Berghoff, director of the institute, said in an interview after learning of the new data.