The chief audit executive (CAE) is responsible for ensuring that the conclusions reached by the external service provider are adequately supported. While the audit committee approves the audit plan and the CEO may approve the engagement of external service providers, it is the CAE's responsibility to oversee the entire audit process, including the quality and substantiation of the conclusions and recommendations made by the external auditors. Reference:
The IIA's International Standards for the Professional Practice of Internal Auditing (Standards), specifically Standard 1312 - External Assessments.
The IIA's Practice Guide on Outsourcing and Cosourcing of Internal Audit Activities.