A spokeswoman for Shelby said the letters were not sanctioned by the committee.
Since the financial crisis, the world’s largest banks have paid billions of dollars to settle broad probes into market manipulation, money laundering and violations of international sanctions.
Earlier this year, for example, JPMorgan, Barclays Plc, and Royal Bank of Scotland Group Plc were among firms that pleaded guilty to manipulating currency markets. Those cases followed an investigation into interest-rate rigging that has already led to settlements with a number of banks.