BAD AROLSEN, Germany — Within weeks of Hitler's 1933 rise to power, the iron gates slammed shut on inmates of the first Nazi concentration camps. It was the start of an unparalleled experiment in persecution and genocide that expanded over the next 12 years into a pyramid of ghettos, Gestapo prisons, slave labor camps and, ultimately, extermination factories.

Holocaust historians are only now piecing together the scattered research in many languages to understand the vast scope of the camps, prisons and punishment centers that scarred German-ruled Europe, like a pox on the landscape stretching from Greece to Norway and eastward into Russia.

Collecting and analyzing fragmented reports, researchers at the U.S. Holocaust Memorial Museum say they have pinpointed some 20,000 places of detention and persecution — three times more than they estimated just six years ago.

And soon they will know much more.

They are about to have their first access to millions of documents locked away for a half century in the sprawling archive of the International Tracing Service, an arm of the International Committee of the Red Cross, in the central German resort town of Bad Arolsen.

The 11 countries governing the ITS have agreed to lift the ban on research that had been imposed to safeguard victims' privacy, though it still will take months until each country ratifies the decision and the doors open.

Recently, The Associated Press was allowed to view ITS documents on condition it did not publish the identities of individuals. Among the yellowed pages and pasted telegrams seen by AP were internal communications, secret orders and "conduct reports" that determined whether inmates would be freed.

The "pyramid" ranged from death camps such as Auschwitz at the top, to secondary and tertiary detention centers. There were 500 brothels, where foreign women were put at the disposal of German officers, and more than 100 "child care facilities" where women in labor camps were forced to undergo abortions or had their newborns taken away and killed — usually by starvation — so the mothers could quickly return to work.

The earliest prisoners were communists, Social Democrats, Jehovah's Witnesses and other political opponents, as well as homosexuals and common criminals. The Final Solution, which ultimately would claim 6 million Jewish lives, had not yet begun.

Survivors have described the camps in agonizing detail, recounting unbearable suffering and calculated brutality. But historians have long sought to know more about the inner workings of the camps, hoping to draw on the Germans' own firsthand accounts and paperwork.

One directive seen by the AP, from November 1943 and marked Private and Confidential, instructed all camp commanders to keep visitors away from sensitive sites.