Moody's Investors Service yesterday placed the triple-A credit rating of Credit Suisse on review for possible downgrade, the first time a major rating agency has ever characterized one of the three major Swiss banks as less than top-notch.

A downgrade would affect about $1 billion in municipal debt backed by Credit Suisse letters of credit, according to Moody's. In a statement, Moody's gave a generalized rationale for the negative review, saying only that Credit Suisse is suffering from "the increasingly competitive banking environment in Switzerland and abroad."

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