If the customer deposits a check that turns out to be fake, I would file a SAR if the amount is above $5,000. I would use the information from the customer's letter for your suspect.
The unknown suspect usually happens when you suffer a loss from a forged or counterfeit check. In those instances, you don't know who the suspect is.
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CRCM,CAMS
Regulations are a poor substitute for ethics.
Just sayin'