The litigation shines a critical light on the Federal Reserve’s on-again-off-again power to bail out nonbanking institutions like Wall Street firms and insurance companies. The Fed first got that authority during the Great Depression, but Congress revoked it in 1958. And then, as the legal walls between banking and other financial services began to fall in the 1990s, Congress once again gave the Fed the power to make emergency loans to nonbanks.

The relevant language is contained in a single, murky sentence inserted in a bill passed the day before Thanksgiving in 1991, as members of Congress were rushing to catch their flights home. Former Senator Christopher Dodd added it at the request of Goldman Sachs and other Wall Street firms, which were still stinging from a major market crash in 1987 and eager to empower the Fed to step in if a similar problem happened again.

The Casadys’ lawsuit says the resulting law needs judicial review because it went flying through Congress with little debate and now appears to be feeding high-risk behavior. Investors in nonbanks now expect that the Fed will open a safety net to catch them, should they falter, the suit contends.

“Congress did not show a legislative intent to convert the Federal Reserve into a bank for bailing out failed speculators,” the complaint asserts.

The suit was filed under the False Claims Act, a federal law that permits private citizens to sue on behalf of government agencies if they believe they have knowledge of a fraud. The law gives people a chance to try to recover money for the government and, by extension, the taxpayers. Plaintiffs who succeed typically get a percentage.

Although the bailout of A.I.G. took place over many months and involved a total commitment of $182 billion, the lawsuit focuses on just part of it — two emergency loans, totaling about $44 billion, made at the end of October 2008. The money was used to settle trades involving two big blocks of complex, mortgage-linked securities, some of which were underwritten by Goldman Sachs and Deutsche Bank, and guaranteed by A.I.G.

When A.I.G. went into a free fall in 2008, the Fed extended the two loans to buy up the troubled securities and put them into two special-purpose vehicles, Maiden Lane II and Maiden Lane III, for holding until the turmoil subsided. Earlier this year, the Fed allowed some of the impaired assets to be sold to undisclosed purchasers.

The Casadys say the Fed erred in making the loans, because it needed a pledge of high-quality collateral from A.I.G. and instead got a big portfolio of impaired assets.