Given the urgency and the lack of internal expertise in forensic investigation, the most effective and immediate solution is to outsource the investigation to independent professional consultants. This approach ensures that the investigation is conducted by individuals with the necessary skills and experience, thereby maintaining the integrity and quality of the investigation. Training internal staff or recruiting new auditors would take time and may not address the immediate need, while declining the engagement would not fulfill the audit committee's request. Reference:
"Internal Auditing: Assurance & Advisory Services" (The Institute of Internal Auditors)
"Forensic Accounting and Fraud Investigation for Non-Experts" (Howard Silverstone and Michael Sheetz)