The Nazis began liquidating the lower camp, and their father, Jacob, a gluemaker from Radom, Poland, told his sons it was time for them to leave him and save themselves.
No, they said. They'd be back soon with something to eat, they promised. That day, April 28, 1945, was the last time they saw their father. The Nazis swept through the lower camp and led their captives on a forced march. Four days later, the Americans arrived.
Sol Finkelstein never learned how or where his father died. He could never mark the anniversary of his passing with prayer, as is the custom of his Jewish faith.
It has taken Sol's son, Joe, a lawyer from Bala Cynwyd, more than three years of combing archives in Washington, Israel, Poland, Austria, and Germany to piece together what happened to his grandfather so he could lift his father's burden.
Starting in a few weeks, others will be able to conduct similar research in a matter of seconds.
For several decades, records from the Nazi war machine were stored in Bad Arolsen, Germany, and managed by the International Red Cross. The public had no access.
In 2007, the Holocaust Memorial Museum received 120 million individual documents from the German archive - freshly digitalized and indexed. The museum staff realized how important it was to put the rest of its 50 million records online while survivors were still alive.
For the last three months, nearly 2,000 volunteers have been entering names, dates, and places into their computers, using software donated by Ancestry.com. (To help, go to worldmemoryproject.org.)
On his own, Joe was able to learn that his grandfather had survived the death march, and that the Allies had taken him to a hospital in Wels, Austria, where he died of typhus four days after liberation.
"He had care," he said. "He had doctors. My father could not have helped him."
In 2009, Joe took his family to Austria for a graveside ceremony. Sol was too sick to travel.
The chief rabbi of Austria, Paul Chaim Eisenberg, said a blessing, the family recited Kaddish, the funeral prayer, then the rabbi turned to the camera, with which the Finkelsteins were documenting the event for Sol to watch later.
"Your father was a hero," the rabbi said. It was God's fate "that you and your brother should survive this terrible Holocaust."
Joe took a picture of his grandfather's headstone, which Sol later placed by his bedside. He'd wake up and talk to the photograph, and then again at night. That memento was all of his father that survived the war.
Then, this spring, a researcher from the Holocaust museum called Joe. Searching their archives, they'd found his grandfather's identification card from the Radom ghetto. With a photo.
Sol has put that next to his bed. And he's put another copy on the coffee table by the TV, which he watches during the day. "My grandfather's still dead, but it's a different kind of grief we feel," Joe said. "To have lost someone and not to be able to find them. . . . Now he's more than a name and a thought. He's a real person with a face."
Contact columnist Daniel Rubin at 215-854-5917, firstname.lastname@example.org
or @danielrubin on Twitter. Read his blog at philly.com/blinq