Inquiries into Moody's are in the early stages, largely because state and federal authorities have dedicated more resources to the S&P lawsuit, the sources said, speaking off the record.
Moody's spokesman Michael Adler and Justice Department spokeswoman Adora Andy declined to comment.
Moody's has defended itself against similar allegations in the past, including a 2011 congressional report that concluded the big ratings agencies manipulated ratings to drive business.
Moody's said it takes the quality of its ratings and the integrity of the ratings process very seriously. It also said the firm has protections in place to separate the commercial and analytical parts of its business.
The U.S. Justice Department filed a $5 billion lawsuit against S&P late Monday night and accused it of an egregious scheme to defraud investors in the run-up to the financial crisis that was fueled by a desire to gain more business.
Shares of McGraw Hill Cos. Inc. (NYSE:MHP), which owns S&P, have fallen more than 25 percent since news of the lawsuits. Moody's shares have fallen about 15 percent, even though it was not named in any of this week's actions.
"Don't think Moody's is off the hook," one law enforcement official said.
Another rival, Fimalac SA's (Paris:LBCP) Fitch Ratings, is unlikely to face similar action, the sources said, since it is a much smaller player in the U.S. ratings industry. It also escaped much scrutiny from congressional investigators.
S&P says the lawsuit is meritless and said it will vigorously defend itself.
A similar coordinated federal-state action against Moody's would follow lawsuits two states have already filed. Connecticut, which led the states in this week's actions, sued Moody's and S&P in March 2010.
In January a state court in Hartford denied the last of the preliminary motions Moody's had filed to have the case thrown out. That case and the one against S&P are proceeding to trial in the second half of 2014.
Sen. Richard Blumenthal, D-Conn., who as state attorney general brought the cases against S&P and Moody's in 2010, said he found rampant abuse across the credit rating industry.
"The difference is one of degree and scale rather than essential modus operandi," Blumenthal said in an interview. "S&P is the largest and they did the most sizeable amount of ratings with the largest profits."
Those earlier cases and the more recent ones against S&P are based on a theory that the firms misled investors by stating that their ratings on mortgage products were objective and not influenced by conflicts of interest.