A Suspicious Activity Report (SAR) is a document that financial institutions must file with the Financial Crimes Enforcement Network (FinCEN) under the Bank Secrecy Act when they detect transactions that may indicate illegal activity. Banks file over 3 million SARs annually. The threshold for filing is any transaction of $5,000 or more that a bank has reason to suspect involves illegal activity. SARs are confidential and law enforcement can use them to investigate financial crimes.