I don’t know why anybody should be surprised that rumors fly while such events are in progress and immediately afterward, and that a very great many of them turn out to be inaccurate.
I believe in a slightly different context it’s called “the fog of war.”
In the MSM it’s made much worse by a desperate need to have something to report, and even more to be the first to report it. Any outlet that acted responsibly and reported that they really didn’t know yet would immediately lose its audience to one that was reporting something, even if it later turned out to be wrong.
The whole Sandy Hook story is “The Fog of Propaganda.”