whistle-blower

whistle +‎ blower, referring to a police officer blowing their whistle on observing a violation of the law.

whistle-blower (plural whistle-blowers)

1. (idiomatic) One who reports a problem or violation to the authorities; especially, an employee or former employee who reports a violation by an employer.

   We owe it to a small group of brave whistle-blowers that we know about the infractions at all.

• Thesaurus:informant

• whistleblowing

• blow the whistle
• tattletale