In a press release, the New York-based rating agency said it considered serious the allegations that the company steered customers to insurers with whom it had payoff agreements and arranged for fake bids when it shopped the policies, but said it was too early to say how it would affect the company.

If Marsh remains the sole target of Mr. Spitzer's legal action, then the charges will damage the company more severely than if other brokers are brought under the scope of the investigation, Moody's said.

Prudential, Goldman Sachs and Keefe Bruyette Woods all took negative actions on Marsh stock today in reaction to the announcement.

New York-based American International Group, cited in the lawsuit filed by the New York attorney general's office as one of the insurers participating in the alleged scheme, saw its stock, a component of the Dow Jones Industrial Average, open seven percent lower this morning.

In a research note issued this morning by Prudential Equity Group, analysts said "the equivalent of $18 billion being shed from the AIG Market Capitalization, we are inclined to think that the market has overreacted."

"Still we doubt the issues raised by the New York attorney general will subside quickly and may hamper a recovery in AIG shares in the near-to-medium term," Prudential wrote.

AIG was named in the complaint, but no charges were filed against the company.

As to Marsh, Prudential lowered its rating to underweight from neutral weight.

In a note to investors this morning, Keefe Bruyette Woods said that small commercial insurers as well as personal lines insurers will emerge relatively unscathed from whatever scandal unfolds.

"Among the insurers, we could see dislocation, without the benefit of increased pricing that typically accompanies such dislocation, as well as increased regulation and transparency," the note said.

KBW maintained its market performance rating on the company but lowered its price target to $35 from $47.