The heirs of Nazi-era Jewish art dealers say they have filed a lawsuit in the U.S. suing Germany and a German museum for the return of a medieval treasure trove worth an estimated $226 million.

The suit, which attorneys said was filed late Monday in the U.S. District Court in Washington, D.C., is the latest salvo in a long-running campaign by the heirs for return of the so-called Welfenschatz, or Guelph Treasure — which they claim their ancestors sold under Nazi pressure.