Which of the Gospels were written by eyewitnesses and why?

- Were the Gospels written by eyewitnesses? Which ones? What do scholars & historians have to say on this? - What are the best arguments for the eyewitness authorship of (some of) the Gospels? - Even if (some of) the Gospels were not written by eyewitnesses, can the content still be traced back to actual eyewitnesses? In other words, can we confidently conclude that most of the content of the Gospels is ultimately rooted in the testimony of actual eyewitnesses? If so, why? ___ **Note**: this question is related to several others (see below), although the focus here is on the *eyewitness* condition of the potential authors or of their ultimate sources: - https://christianity.stackexchange.com/questions/15442/are-the-gospel-authors-who-we-believe-them-to-be - https://christianity.stackexchange.com/questions/83374/who-actually-were-writers-of-the-gospels - https://christianity.stackexchange.com/questions/43190/among-modern-critics-who-believe-in-the-attributed-authorship-of-the-gospels-wh Another closely related question is this one: https://christianity.stackexchange.com/questions/83320/is-it-correct-to-say-bible-has-2-eyewitness-accounts-matthew-mark-and-luke-as . Unfortunately, the only [answer](https://christianity.stackexchange.com/a/83339/50422) so far simply affirms that *"the claim is true: we have two Gospels written by eyewitnesses, those of Matthew & John"* but doesn't provide a justification as to why. My question has an explicit focus on **why**: **how do we know** that they were eyewitnesses? (Claims need to be substantiated, hence the *apologetics* tag. If it serves as context, I'm asking this as a follow-up to [this question](https://christianity.stackexchange.com/q/89886/50422).)