The question in my mind is whether there were any warning signs in the picture that might/should have been noticed and acted upon before this happened. Not saying there had to have been, just wondering if there were.

"How could this happen" isn't really the question. Put 100k people under intense pressure for enough time, the laws of probability say sooner or later somebody snaps, no matter how well selected the group is. That makes it all the more important for people to keep watch for any indication that something's not right. Not meant to excuse or justify in any way, just saying that it's a possibility that has to be accepted and looked out for when people are placed in these positions.