This is a difficult question. The United states made great efforts to account for every one of our soldiers who was thought to have been captured. When the US got control of prison camps I am certain they cataloged every POW there. But I don't know if they cataloged civilians. It is highly likely they did but civilians may not have been included in the POW lists that were published later. If you know which camp you could make an inquiry with the US National Archives. Google "NARA". If your father-in-law was held at another concentration camp and/or was repatriated by the Red Cross he should have a file at the the Bad Erolsen Archive in Germany. The Bad Erolsen Archive is the repository for all the surviving records from the German concentration camp sytem. It has it's own rather demanding rules and I have never had any experience with them.