Bloomberg reported that in just six months, Standard & Poor’s AAA blessing for a slice of a $1.5 billion collateralized debt obligation (CDO) cost Citigroup its $15 million investment.

Bloomberg said Citigroup’s Citibank unit bought into what was supposed to be the safest portion of the CDO, relying in part on S&P’s assessment of the securities, according to the complaint. It lost the entire investment when the CDO defaulted on October 19, 2007.

Bloomberg noted that the CDO, underwritten by UBS AG, is one of dozens of deals listed in the DOJ’s lawsuit against S&P and its parent McGraw-Hill that received the ratings firm’s highest AAA grade.

To read the Bloomberg article click here.