The note is on a small, yellow piece of paper, written out in thin letters, all capitals. It is pinned to the door of the modest rowhouse in Northeast Philadelphia, where enough visitors have come in recent days to warrant a pointed warning:

"WE DO NOT HAVE ANY COMMENT," it reads. "PLEASE LEAVE."

The resident, a retired toolmaker named Johann "Hans" Breyer, has lived on this well-kept block for 3 1/2 decades, according to property records. His past, as a guard at Auschwitz during World War II, again came back to haunt him last month when authorities in Germany announced that he is under investigation....