Acme...
I'm not a fan of reporting on suspected income tax evasion; I can give anyone 15 - 20 minutes on why it makes no sense and would love a national forum to do so.
Like you, I find no specific reference in the regulations or the instructions to the form. However, the Handbook does make one oblique reference to it saying: "...banks are not obligated to investigate or confirm the underlying crime (e.g., terrorist financing, money laundering, tax evasion, identity theft, and various types of fraud).
Moreover, I've heard people from the top of the food chain at FinCEN insist that suspected income tax evasion is reportable... Mind Kaybee's point, but any alternative explanation I see is purely theoretical and if the borrower didn't suggest it, I certainly wouldn't assume it.
You may get some push back in your bank. So, I suggest you call the FinCEN Helpline and ask. Don't ask if you should file the SAR because they will not answer the question. Just ask if suspected income tax evasion is reportable. If there are people in your bank who do not want the SAR filed then that leaves it to them to explain why this is not income tax evasion.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.