This whole thing goes into risk assessment and who you can really trust. AIG thought it could trust a big bank like Goldman and their rating agencies, but in reality they could not. We're talking billions of dollars of packaged loans, it's practically impossible to go through and review every loan in a sensible time. That fact was obviously a key part of the con job, but it's also entirely possible that AIG was well aware of what was happening. Only when everything exploded the nitty gritty of unraveling every piece of paper happened.
What's striking about all this is that the vast majority of actors that took part on this con simply paid off a monetary penalty and walked away scot-free without admitting to any wrongdoing or facing criminal charges, despite the seriousness of the situation.