Whatever agency is responsible. I dont know what that would be called in your country.
But if you were a psychiatrist and a patient told you that he was abusing children, there is some official body you could tell. They would make him stop lock him up etc. The same would be true with murder, you would go to the police. The people that need to be told may differ depending on what he is doing.
They use 'authorities' as a generic term to cover all such official agencies.
Student or Learner