An organization experienced a sophisticated phishing attack that resulted in the compromise of confidential information from thousands of user accounts. The threat actor used a land and expand approach, where initially accessed account was used to spread emails further. The organization's cybersecurity team must conduct an in-depth root cause analysis to uncover the central factor or factors responsible for the success of the phishing attack. The very first victim of the attack was user with email 500236186@test.com. The primary objective is to formulate effective strategies for preventing similar incidents in the future. What should the cybersecurity engineer prioritize in the root cause analysis report to demonstrate the underlying cause of the incident?