What caused the sudden bank shutdown in Santa Anna National Bank?

The sudden shutdown of Santa Anna National Bank was caused by a combination of factors, primarily involving fraudulent activities and unsafe banking practices:

1. **Suspected Fraud**: Fraudulent activities played a significant role in the bank's failure. The FDIC estimated that the bank's failure would cost $23.7 million to the Deposit Insurance Fund. This cost is likely to change as assets are sold, according to the regulator.
2. **Substantial Dissipation of Assets and Earnings**: The bank experienced a significant dissipation of its assets and earnings due to unsafe or unsound practices. The OCC found that the bank was in an unsafe or unsound condition to transact business, and its assets were less than its obligations to its creditors and others.
3. **Exceeding FDIC Insurance Limits**: Approximately $2.8 million of the bank's deposits exceeded FDIC insurance limits. This excess amount contributed to the bank's instability and ultimately led to its closure.
4. **Operational Inefficiencies**: The bank's substantial dissipation of assets and earnings, coupled with the fraudulent activities, led to operational inefficiencies and a critical undercapitalization, which were untenable in the long run.

In summary, the sudden shutdown of Santa Anna National Bank was primarily caused by fraudulent activities, substantial dissipation of assets, and operational inefficiencies that rendered the bank insolvent and forced its closure by regulatory authorities.