Enforced by the US Department of the Treasury (Financial Crimes Enforcement Network -FinCEN)

http://www.fincen.gov/whatsnew/html/sar_faqs.html

A suspicious activity report (or SAR) is a report made by a financial institution to the Financial Crimes Enforcement Network (FinCEN), an agency of the US Department of the Treasury, regarding suspicious or potentially suspicious activity. SARs are also used by the the UK's Serious Organized Crime Agency.

FinCEN requires a SAR to be filed when the financial institution suspects: insider abuse by an employee; violations of law aggregating over $5,000 where a subject can be identified; violations of law aggregating over $25,000 regardless of a potential subject; transactions aggregating $5,000 or more that involve potential money laundering or violations of the Bank Secrecy Act; computer intrusion; or when a financial institution knows that a customer is operating as an unlicensed money services business. See also Bank Secrecy Act and FinCEN entries.

As of April 1, 2013, financial institutions must use the new FinCEN reports, which are available only electronically via: the BSA E-Filing System FAQs Regarding the FinCEN Suspicious Activity Report (SAR)

US financial institutions, US businesses and US residents.

http://en.wikipedia.org/wiki/Suspicious_activity_report