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

The Vertical ABS 2007-1, a mezzanine cash flow structure built with asset-backed securities underwritten by UBS AG, is one of dozens of deals listed in the Justice Department’s Feb. 4 lawsuit against S&P and its parent McGraw-Hill Cos. that received the ratings firm’s highest AAA grade.