Bloomberg News reports that Connecticut's attorney general has subpoenaed McGraw Hill (NYSE: MHP)'s Standard & Poor's related to an investigation into credit rating.

The legal question is whether S&P issued overly rosy credit ratings on subprime mortgage-backed securities (MBSs) that later lost more than half their value. S&P and its peers were competing for the lucrative ratings fees paid by the investment banks that underwrote the MBSs. And it has been alleged that the winning firms offered the highest ratings.

If the MBSs were indeed the financial equivalent of toxic waste -- then the AAA ratings that S&P and its peers awarded to the MBSs could be thought of as golden wrapping paper. This gold wrap was particularly useful to the investment banks when they visited MBS customers in Europe and Australia.

Rather than get into the details of what was inside the gold-wrapped boxes, these customers simply relied on the assurances of the MBS sales people that the "investment" had been blessed by S&P. No need to dirty themselves with unwrapping the box and exposing the toxic waste within.

Peter Cohan is President of Peter S. Cohan & Associates. He also teaches management at Babson College and edits The Cohan Letter. He has no financial interest in the securities mentioned in this post.