The FFIEC BSA Manual states:
"SAR Decision Making
After thorough research and analysis has been completed, findings are typically forwarded to a final decision maker (individual or committee). The bank should have policies, procedures, and processes for referring unusual activity from all business lines to the personnel or department responsible for evaluating unusual activity. Within those procedures, management should establish a clear and defined escalation process from the point of initial detection to disposition of the investigation.
The decision maker, whether an individual or committee, should have the authority to make the final SAR filing decision. When the bank uses a committee, there should be a clearly defined process to resolve differences of opinion on filing decisions. Banks should document SAR decisions, including the specific reason for filing or not filing a SAR. Thorough documentation provides a record of the SAR decision-making process, including final decisions not to file a SAR. However, due to the variety of systems used to identify, track, and report suspicious activity, as well as the fact that each suspicious activity reporting decision will be based on unique facts and circumstances, no single form of documentation is required when a bank decides not to file."
A thorough reading of this suggestion will show you what the expectations are. If the bank does not document its investigation and decision, it will be difficult for an examiner or auditor to evaluate the process.
Think of school: "Show your work".