Interrogation of an Apostle

Which means he is either copying it from a source that got it from a source, etc. that witnessed the events/fabricated the story; or, he himself has witnessed/fabricated the account.

In any case of texts this old reporting on traditions that were likely oral before being written down, it is impossible to know whether the author is an eyewitness or just a transcriber/copier of reports available to him. Even when the author claims to have witnessed the event himself, we cannot be sure he actually did, or that his claim of witnessing is not just the result of a direct copy from another text.