(Bloomberg) ? Investors in Marsh & McLennan Cos., Aon Corp. and Willis Group Holdings Ltd., the three largest insurance brokers, may be underestimating the risks of regulatory probes and falling insurance prices, said Keefe, Bruyette & Woods Inc. analyst Cliff Gallant.

Gallant lowered his stock rating today on Marsh and Aon to ?under-perform? from ?market perform,? and cut Willis to ?market perform from ?outperform.? All three companies have rallied since shares plunged in October, when New York Attorney General Eliot Spitzer (pictured) accused Marsh in a lawsuit of rigging bids and taking kickbacks from insurers.

?These stocks have gotten a little ahead of themselves,? Gallant said in an interview. ?Mr. Spitzer has said he?s not done, and the fundamental environment for the industry is deteriorating.?

The companies today face a decline in insurance prices, which will erode commissions, Gallant said. New York-based Marsh, which lost almost half its market value in the days after Spitzer?s suit, may lose clients, while Aon and other brokers may be sued, he said.

Marsh shares fell 35 cents, or 1.1 percent, to $31.10 at 1.22 p.m. in New York Stock Exchange composite trading and have gained 29 percent since touching a six-year low on October 19.

Shares of Chicago-based Aon, which rose 30 cents, or 1.3 percent, to $22.70, have climbed 18 percent since October 19. Willis, which slipped 7 cents to $40.64, has gained 29 percent during that period.

Willis, based in Bermuda, may emerge as ?the big winner? from Spitzer?s probe and has a lower chance of being sued, Gallant said. Willis is the only broker among the three whose shares are higher than they were before Spitzer?s October 14 suit.