"Given the risks posed by cryptocurrencies to banks and their customers, we request that you withdraw OCC Interpretive Letters 1170, 1172, 1174, and 1179 and coordinate with the Federal Reserve and the Federal Deposit Insurance Corporation to develop a comprehensive approach that adequately protects consumers and the safety and soundness of the banking system," the letter said.

The senators also asked a series of questions about how many banks have been approved to engage in crypto activities, what kinds of services those banks provide, the dollar volumes the banks hold tied to crypto and whether any of these banks are involved in other crypto-related activities such as trading derivatives.