Alex Blumberg and NPR correspondent (and "Planet Money" reporter) Dave Kestenbaum examine what went wrong with the credit ratings agencies. When all these financial instruments that brought down our economy—the mortgage backed securities, the derivatives—were originally issued, the rating agencies (Standard and Poors, Moody's and Fitch) gave many of these things their top rating of triple-A. Because of the great ratings, trillions were invested. And then the ratings turned out to be wrong. What happened? (25 minutes)