The operation, co-ordinated by prosecutors from the southern Italian city of Potenza, was carried out by Italian, Swiss and U.S. authorities after a year-long investigation, an Italian police source said.
It began as a investigation into mafia loan-sharking, but gradually expanded as prosecutors used telephone and computer intercepts to unearth evidence of illegal activity surrounding Treasury bonds.
The fake securities, worth more than a third of U.S. national debt, were seized in January from a Swiss trust company where they were held in three large trunks.
The U.S. Embassy in Rome thanked the Italian authorities and said the forgeries were "an attempt to defraud several Swiss banks." It said U.S. experts had helped to identify the bonds as fakes.
Potenza's prosecutor Giovanni Colangelo said an international network "in many countries" was behind the forgeries.