From https://www.c-span.org/video/?c3342718/waxman-greenspan-testimony

Alan Greenspan:

So the problem here is, something which looked to be a very solid edifice, and indeed a critical pillar to market competition and free markets, did break down. And I think that, as I said, shocked me. I still do not fully understand why it happened. And obviously, to the extent that I figure out where it happened and why, I will change my views. And if the facts change, I will change.