What do I do if insurance says my work related injury is not in the line of duty?

If an employee is injured or suffers an illness from a critical incident that occurred while on duty and the insurance denies the claim, did that injury or illness now become non duty related? Even though their doctors and other doctors state it was caused by the critical incident the employee responded too? Example, responding to an officer down which was fatal crash where the officer died. The city said since insurance denied the claim the injury/illness is not in the line of duty, and all rights to In The Line Of Duty Benefits are gone. Thanks.