If the internal audit activity is not primarily responsible for conducting fraud investigations and should refrain from involvement in them, it would still be considered acceptable for internal auditors to evaluate the effectiveness of fraud investigations. This role aligns with the audit's function of providing independent assurance on the effectiveness of risk management and control processes within the organization.References:
IIA standards on the role of internal auditing in fraud risk management, which specify that internal auditors may assess the effectiveness and contribute to the improvement of fraud detection and investigation processes.