There were three parts and he mentioned only two. The BIG BIG problem is, these two parts twist the whole thing without the crucial killing part, that is, the killing on the way to reach Tiananmen Square.

Part I: 10 pm, 6/3 to 3 am 6/4; (killing, missed)
Part II: 3 am - 5 pm, 6/4 (described by Xiao Zhang)
Part III: the daytime of 6/4 (of which the author witnessed about 6 pm and after, when the citizens assauted the soliers)

Without part I, what would you think of those who were witnessed assaulting the soldiers in Part III?

It is true that the author said he did not witness part I, but he did not see what Xiao Zhang had seen either. There should have been much more of his acquaintances who had witnessed Part I.

I was in Beijing at the time. I know that almost every Beijing resident could name a person, directly or indirectly, that witnessed Part I. During that period, the first thing the author, like anyone, would want to learn must be WHETHER there had been the killing and WHERE. Strangely, he did not recall ANYTHING of that part.