The explosion of knowledge followed the end of the Cold War, when scholars could reach into long-hidden sources in Eastern Europe. Some of the increase is explained with the inclusion of smaller camps, which had just a few hundred inmates, and of prisoner-of-war camps which shifted location frequently. Until the encyclopedia project, no one had tried to pull it all together into one comprehensive resource, Megargee said.

"When we dig into the Arolsen files, we'll find that what was for us just a name, just a location, is going to take on some life, because all we've had is some notation somewhere that there was such a place," he said.

More information was disclosed in compensation claims after the Germany government and German industries created a $6.6 billion fund in 2000 to compensate people exploited as wartime slave laborers.

Gunter Saathoff, the director of the German Foundation which administers the fund, said nearly 1.7 million people applied for restitution, including some 8,000 who served as human guinea pigs for Nazi medical experiments. The final payments were being made Dec. 31.

The Washington historians say more than 100 local detention camps were set up in the first months of the Nazi regime in 1933, mainly for political prisoners. By mid-1934 control was centralized under the command of SS chief Heinrich Himmler.

Organized killings began shortly after the war started in late 1939 at so-called "euthanasia sites," where the victims were physically or mentally handicapped people or prisoners no longer capable of work. Estimates say the number reached 200,000.

By 1940, Jews in Poland and Russia were being confined in ghettos. As German men were needed to fight, men and women were brought from Poland, the Netherlands, the Soviet Union and other occupied nations to work in German industries. Labor camps proliferated.

In 1941 the Germans devised the Final Solution to exterminate Europe's Jews. In September, even before the plan was formally approved, the SS began experimenting with gas chambers at Auschwitz, about 40 miles from the Polish city of Krakow.

Auschwitz was the largest of six camps whose primary purpose was to kill Jews at maximum speed. In 1941-42, more death camps were built in Chelmno, Belzec, Sobibor, Treblinka and Majdanek.

By the time Germany capitulated in May 1945 and disbelieving Allied forces marched into the camps, 2.7 million people had been incinerated in the ovens or open pits of these six compounds. Of every three Jews in Europe at the start of the war, two were dead.

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Contributing: Melissa Eddy, Randy Herschaft