Italian police said they had seized $6 trillion in counterfeit U.S. bonds stored in Switzerland that were \"possibly\" to be used to purchase plutonium. Police said eight persons were arrested on charges of international finance fraud, the Italian news agency ANSA reported Saturday. The arrests, after a year-long investigation by Swiss and Italian officials, all occurred in Italy. Police said an \"organization specializing in international financial transactions guaranteed by counterfeit U.S. bonds,\" had moved the bonds from Hong Kong to Zurich, Switzerland, in 2007. The Guardian newspaper said the case was thought to involve the largest ever seizure of counterfeit Treasury bonds. The bonds, dated 1934. were found by Swiss authorities in three large trunks in a Zurich bank. Police in Italy called the fraud a \"severe threat\" to international financial security. Fake bonds are a common mafia scam, the Guardian reported. Often they are used as collateral on loans or to open a line a credit, which the fraudsters tap before disappearing. Police called the one-year investigation \"Operation Vulcanica.\" U.S. Treasury officials in Italy would not comment on the case except to confirm that the bonds were forgeries.