the decision to investigate if alcohol was a contributing factor to an incident rests with what individual

Commanding officer.

Explanation:

Commanding officer can be defined as any individual who works with the armed forces and is in charge of a group or operation in a military unit. They are saddled with the responsibility of providing guidance and acting as a role model or example by being morally upright, confident, positive and dominant to their subordinates.

Hence, the decision to investigate if alcohol was a contributing factor to an incident rests with a commanding officer.

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