Moody's Corp., one of the big three credit-rating companies that have come under fire for their role in the housing collapse, today ousted another of its senior executives and said employees violated ethics guidelines in assigning top ratings to complex securities that subsequently lost as much as 90 percent of their value.

Noel Kirnon, the head of the structured finance unit at Moody's that dealt with mortgage-backed securities, is the second high-profile departure this year. Kirnon's boss, Moody's President and Chief Operating Officer Brian Clarkson, said in May that he was leaving.

Moody's and its competitors, Standard & Poor's and Fitch Ratings, are under investigation by Congress, federal agencies and state attorneys general, who are trying to determine if conflicts of interest led them to rate mortgage-related securities higher than they should have been rated, fueling the credit boom. As explained in The Post's recent series on the housing bubble, the collapse of those securities has added to the growing problems of the financial system and the economic downturn.

Many of the inherent conflicts at the credit rating companies were outlined long before the current crisis, in a 2004 Post series by Alec Klein. He reported on how the raters have become some of the most important gatekeepers in capitalism without the commensurate oversight or accountability.

In the latest development at Moody's, a review by an outside law firm found that some Moody's analysts "considered factors inappropriate to the rating process" when reviewing European constant proportion debt obligations, or bonds backed by derivatives. Employees involved face disciplinary proceedings that may include termination, Moody's says.

The increasing scrutiny of the credit raters has led to some changes. Last month, the companies reached an agreement with the New York state attorney general's office to change the way they evaluate mortgage securities.
And the Connecticut attorney general pressured the raters to improve the way they rate municipal bonds, the Wall Street Journal reported.

Still, many believe that, as in previous financial crises, the raters may escape any serious attempt to change the way their business is regulated.