The person who reported the incident wished to remain anonymous.
There are laws to protect whistleblowers.
内部告発者 specifically refers to an insider whistleblower exposing organizational wrongdoing, whereas 通報者 can be any reporter, including external witnesses.
Whistleblower Protection Act
Compound of 通報 (つうほう, 'report, notification') + 者 (しゃ, 'person').