According to the guidance issued by FinCEN and the Federal banking agencies, when a financial institution receives a request for SAR supporting documentation from FinCEN or an appropriate law enforcement or supervisory agency, it must first verify that the requestor is, in fact, a representative of such an agency. This is to ensure the confidentiality and security of the SAR information and to prevent unauthorized disclosures. A financial institution should have procedures for such verification in its BSA/AML compliance program, which may include, for example, independent employment verification with the requestor's field office or face-to-face review of the requestor's credentials.
References:
1: This web page explains the BSA requirement that financial institutions provide SAR supporting documentation in response to requests by FinCEN and appropriate law enforcement or supervisory agencies, and the need to verify the requestor's identity.
2: This document provides answers to frequently asked questions regarding SARs and other AML considerations, including the question of how to handle "keep open" requests from law enforcement.

Alternative remittance, also known as underground banking or informal value transfer systems, is a method of transferring money or value without using formal financial institutions or channels. It is often used by migrant workers, refugees, or people who lack access to formal banking services. However, it can also be exploited by criminals and terrorists to move funds across borders without detection or regulation. The Wolfsberg Group, a group of leading international banks that promotes best practices in anti-money laundering and counter-terrorist financing, cited alternative remittance as one of the sectors and activities that are widely used for the financing of terrorism in its Statement on the Suppression of the Financing of Terrorism. The Wolfsberg Group recommended that financial institutions apply enhanced and appropriate due diligence to customers engaged in alternative remittance and report any suspicious transactions to the relevant authorities.
References:
Wolfsberg Group (2002). Wolfsberg Statement on Anti-Terrorism Financing1 Pieth, M. (ed.) (2002). Financing Terrorism. Springer, Dordrecht2 ACAMS (2020). CAMS Certification Package (6th Edition)3