Bart D. Ehrman, The Triumph of Christianity: How a Forbidden Religion Swept the World (Simon and Schuster: 2018), 187.
[In his letter to Pliny the Younger concerning how to deal with Christians] Trajan does stress two provisos. Christians were not to be sought out. There was to be no witch hunt, no tracking down of criminals. Second, Trajan emphasizes, accusations could not be made anonymously. If someone had a charge to bring, they needed to bring it in person and stand by their claim: “Documents published anonymously must play no role in any accusation, for they give the worst example, and are foreign to our age.” With that, Trajan’s reply ends.
It should be clear that there were simply no established procedures for dealing with Christians in the early second century. Pliny’s approach was cleaver and effective, but it was also ad hoc. He was not following precedent, because he did not know any precedents.