How did Wall Street get flawed mortgage securities past raters? Reverse engineering

One of the mysteries of the financial crisis is how mortgage investments that turned out to be so bad earned credit ratings that made them look so good.

One answer is that Wall Street was given access to the formulas behind those magic ratings — and hired away some of the very people who had devised them.

In essence, banks started with the answers and worked backward, reverse-engineering top-flight ratings for investments that were, in some cases, riskier than ratings suggested, according to former agency employees.

The major credit rating agencies, Moody’s, Standard & Poor’s and Fitch, drew renewed criticism on Friday on Capitol Hill for failing to warn of the dangers posed by complex investments like the one that has drawn Goldman Sachs into a legal whirlwind.

But while the agencies have come under fire before, the extent to which they collaborated with Wall Street banks has drawn less notice.

The rating agencies made public computer models that were used to devise ratings to make the process less secretive. That way, banks and others issuing bonds — companies and states, for instance — wouldn’t be surprised by a weak rating that could make it harder to sell the bonds or that would require them to offer a higher interest rate.

But by routinely sharing their models, the agencies in effect gave bankers the tools to tinker with their complicated mortgage deals until the models produced the desired ratings.