Screening customers at onboarding and quarterly thereafter is not sufficient to ensure compliance with economic sanctions, as sanctions lists may change frequently and the financial institution may not be aware of the latest updates. Screening should occur promptly after list updates to ensure that the financial institution is not dealing with a sanctioned individual or entity, or facilitating a prohibited transaction. This is recommended by the international guidance from the Financial Action Task Force (FATF) and the Wolfsberg Group12.
Screening and performing enhanced due diligence on new relationships is also important, but not the only step to ensure compliance.
References:
CAMS Certification Package - 6th Edition | ACAMS, Chapter 3: Sanctions, page 86 The Wolfsberg Group Correspondent Banking Due Diligence Questionnaire 2014, Section 5: Sanctions Policy, page 12 ACAMS CAMS Certification Video Training Course - Exam-Labs, Video 3.1: Sanctions Exam CAMS: Certified Anti-Money Laundering Specialist (the 6th edition), Question 109 The European Union Fourth Anti-Money Laundering Directive (4th AMLD) is a legal framework that aims to prevent the use of the Union's financial system for the purposes of money laundering and terrorist financing.
One of the provisions of the 4th AMLD is to lower the currency threshold for cash payments from €15,000 to
€10,000. This means that any person who makes or receives cash payments of €10,000 or more, whether in a single transaction or in several linked transactions, is subject to customer due diligence and record-keeping obligations. The 4th AMLD also extends its applicability to providers of gambling services, which are now listed as 'obliged entities'.
References:
Directive - 2015/849 - EN - Fourth Anti-Money Laundering Directive - EUR-Lex, Article 11 and Recital 23.
EUR-Lex - 02015L0849-20210630 - EN - EUR-Lex, Article 11 and Recital 23.
Key elements of the 4th EU Anti-Money Laundering Directive, Section: Cash payments.
Anti-money laundering and countering the financing of terrorism legislative package, Section: New EU AML/CFT Regulation.

The Certified Anti-Money Laundering Specialist (the 6th edition) Study Guide states that financial institutions should address all requests from law enforcement agencies completely and in a timely manner (Page 83).
Disregarding requests when there is a justifiable reason for doing so is not recommended, as this could impede the investigation. Sharing information about the investigation with analysts is not recommended, as this could compromise the investigation. Delaying responses by informing senior management of requests is also not recommended, as this could result in a delay in the investigation.

According to the Basel Committee's principles on customer due diligence, banks must obtain the necessary information to establish the identity of their customers, beneficial owners, and any persons acting on behalf of their customers. This includes verifying the identity of the customer and the beneficial owners and assessing the customer's risk profile. Additionally, banks must conduct ongoing due diligence to ensure that the customer does not give rise to suspicions of money laundering or terrorist financing [1].