Italian police said on Friday they had seized about $6 trillion of fake U.S. Treasury bonds in Switzerland, and issued arrest warrants for eight people accused of international fraud and other financial crimes.

The operation, coordinated by prosecutors from the southern Italian city of Potenza, was carried out by Italian and Swiss authorities after a year-long investigation, an Italian police source said.

The fake securities, 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 eight alleged fraudsters are accused of counterfeiting bonds, credit card forgery, and usury in the Italian regions of Lombardy, Piedmont, Lazio and Basilicata, police said.

The Swiss Federal Prosecutor's office said Zurich state prosecutors had worked on the investigation at the request of the Italian prosecutor. The Swiss handed over their findings in July of last year.

In 2009, Italian financial police seized $742 billion of fake U.S. bearer bonds in the northern Italian town of Chiasso, near the Swiss border.