The Bank Secrecy Act of 1970 (or BSA, or otherwise known as the Currency and Foreign Transactions Reporting Act) requires financial institutions in the United States to assist U.S. government agencies to detect and prevent money laundering.
Specifically, the act requires financial institutions to keep records of cash purchases of negotiable instruments, and file reports of cash purchases of these negotiable instruments of more than $10,000 (daily aggregate amount), and to report suspicious activity that might signify money laundering, tax evasion, or other criminal activities.
http://en.wikipedia.org/wiki/Bank_Secrecy_Act
What I find interesting about this whole thing is that inflation has made the amount so relatively small. Back in 1970, $10,000 would get you a REALLY NICE new car. Or a new house.
IOW, the amount does not have to be 10K, just something that warrants “suspicion”.
Interesting side bar, one bank that did not adhere to these directions was Chase Manhattan when it was headed by David Rockefeller. They refused to go along with the regs at that time. Coincidently, his brother was the VP?