My brother found a reference to The International Tracing Service, at Bad Arolsen, Germany. This is the largest archive of Holocaust related documents. Here is the description from Wikipedia:
The International Tracing Service (ITS) in Bad Arolsen, Germany, is the internationally governed archive whose task it is to document the fate of millions of civilian victims of Nazi Germany. The documents in the ITS archives include original records from concentration camps, details of forced labour, and files on displaced persons. ITS preserves the original documents and clarifies the fate of those persecuted by the Nazis. Since November 2007, the archives are accessible for researchers.
ITS was founded in 1943 as an organization dedicated to finding missing persons, typically lost to family and friends as a result of war or political unrest during World War II. The service operates under the legal authority of the Bonn Agreement, is under the administrative umbrella of the International
Committee of the Red Cross and is funded by the government of Germany.
ITS’s total inventory comprises 26,000 linear metres of original documents from the Nazi era and post-war period, 225,000 meters of microfilm and more than 100,000 microfiches. Work is underway to digitize the files, both for purposes of easier search and for preserving the historical record.
The documents in the archive were only made available to the public in 2007, after years of controversy. I remember hearing about this, and wondering if there might be some information there on our missing relatives. Although the documents have been digitized, it is still not possible to access them on-line. (This too has been a source of controversy, angering survivors and families.) But it is possible to fill out a form and request that the ITS staffers search for evidence of your family members. My brother found this form on-line, and I filled it out for the family members whose names I know - Leibl Korenbaum, Reizel Korenbaum, Liba Waszerstrom, Reuven Zylberkant, David Zylberkant. The forms request details about the person's life (such as date of birth, last address, married or single, number of children), for most of which I had to fill in "unknown". You also must certify that you are a family member of the person you are looking for, and if not, send a power of attorney form authorizing you to access the information.
It said it would be 6-8 weeks before our query is answered, and any documents will be sent to the address I gave. So now we wait.