The recent startling recovery of long-lost artworks made for Adolf Hitler and his chief architect, Albert Speer, began with a telephone call to a Berlin art dealer.

Two large and imposing bronze horses by Josef Thorak — missing from a Soviet military base outside Berlin since some point in communism’s collapse — were available. Was the dealer, Traude Sauer, interested?

Ms. Sauer, 76, who by her own estimation is a dealer of distinction, has long been a police informant. Realizing that the Nazi-era sculptures might be classified as stolen state property, she turned to René Allonge, a chief investigator with the Berlin police.