"Once you get on the government's radar, it's hard to get off scot-free," Robinson said.
The government charged S&P under a law intended to make sure banks invest safely. If S&P is found to have committed civil violations, it could face not only fines but also limits on how it does business.
McGraw-Hill shares closed down nearly 11 percent Tuesday. On Monday, after the lawsuit was first reported, they plunged nearly 14 percent.
Shares of Moody's Corp., the parent of Moody's Investors Service, another rating agency, lost nearly 9 percent Tuesday after closing down nearly 11 percent Monday.
Christina Rexrode reported from New York. AP writers Pete Yost and Marcy Gordon in Washington and AP Business Writer Bree Fowler in New York contributed to this report.
Daniel Wagner can be reached at www.twitter.com/wagnerreports .
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