Packaging Mortgages into Securities

I believe before the mortgage meltdown investment banks used to purchase mortgages from mortgage banks, package them and later sell them as a security. But prior to the sale as a security, it would hire a firm (e.g., Clayton Holdings) to evaluate the loans. That firm (e.g., Clayton Holdings) would produce a document summarizing its findings (i.e., amount and detail on bad loans). I believe that document was made available to prospective purchasers of the securities.

Why didn't the investment banks turn back the bad mortgages to the mortgage banks?