Italian prosecutors say they have broken up an organised crime ring that was hiding trillions of dollars of fake US bonds.
The bonds, with a face value of $6tn, were found in three metal boxes in a warehouse in the Swiss city of Zurich.
Italian authorities have arrested eight people and are investigating them for fraud and other crimes.
Prosecutors are not sure what the gang was planning, but think they intended to sell the counterfeit bonds.
Investigators, based in Potenza in southern Italy, say the fraud posed "severe threats" to international financial security.
In cooperation with Swiss police, they tracked down three metal boxes to a warehouse in Zurich. The crates contained thousands of fake US bonds that gave the appearance they had been issued by the US Federal Reserve in 1934.
US officials confirmed that the bonds were counterfeit.
Fake US securities have been seized in Italy before and there were at least three cases in 2009.
But this case is on a different scale to previous investigations, as the fake bonds have a value equivalent to almost half of the entire US debt pile.
"Everything began with an investigation into mafia clans in the Vulture-Melfese area in the southern Basilicata region," said Giovanni Colangelo, the head of the prosecutor's office in Potenza.