This is what information I could find on an OFAC 602 Letter. Is it possible your FI processed a transaction for a prohibited party? Also, while this is only my interpretation, it looks like you must respond to this inquiry. However, I would definately confirm this with Bank Counsel.
When it comes to OFAC’s attention that an illicit transaction was
processed through a U.S. bank, without being blocked or rejected, as
appropriate, OFAC normally sends an administrative demand for
information, called a “602 letter,” to the bank requesting an explanation
of how the transaction was processed. Upon receipt of the bank’s
response to this letter, the case may be referred to the Civil Penalties
Division, which issues a “Prepenalty Notice” citing the violation and
stating the amount of the proposed penalty. The bank then has thirty days
to make a written presentation as to why a penalty should not be imposed,
or, if imposed, why it should be in a lesser amount than proposed. It is
critical for banks to answer such “Prepenalty Notices” since failure to
respond may result in default judgements levying maximum fines.
Mitigating factors in Civil Penalty procedures include self-disclosure,
the use and sophistication of interdict software, and other bank
compliance initiatives. TWEA civil penalty and forfeiture proceedings
include the opportunity for an administrative hearing and pre-hearing
discovery prior to imposition of a penalty or forfeiture.
2 September 28, 2006 Department of the Treasury
"100 victories in 100 battles isnt the most skillful. Subduing the other's military w/o battle is the most skillful." Sun-Tzu