1. An accident investigation aims to determine the root cause or underlying factors that led to an accident by examining failures in people, equipment, supplies or surroundings. 2. When investigating an accident, the investigator should ask "why" at least five times to dig deeper for the root causes, working backwards from the accident to reveal the underlying cause. 3. A thorough accident investigation includes interviewing witnesses, documenting the accident scene, gathering supporting documents, and analyzing the pre-accident conditions, accident sequence, and post-accident conditions to identify the root cause and prevent future occurrences.