The inquiries will focus on the financial institutions and the individuals that ran them, the senior law enforcement official said.
The law enforcement officials spoke on condition of anonymity because the investigations are ongoing and are in the very early stages.
Officials said the new inquiries bring to 26 the number of corporate lenders under investigation over the past year.
Spokesmen for AIG, Fannie Mae and Freddie Mac did not immediately return calls for comment Tuesday evening. A Lehman spokesman did not have an immediate comment.
Just last week, FBI Director Robert Mueller put the number of large financial firms under investigation at 24. He did not name any of the companies under investigation but said the FBI also was looking at whether any of them have misrepresented their assets.
Over the past year as the housing market cratered, the FBI has opened a wide-ranging probe of companies across the financial services industry, from mortgage lenders to investment banks that bundle home loans into securities sold to investors. Mueller has previously said the FBI's hunt for culprits in the nation's subprime mortgage crisis focused on accounting fraud, insider trading, and failure to disclose the value of mortgage-related securities and other investments.
The investigations revealed Tuesday come as lawmakers began considering whether to approve emergency legislation that would give the government broad power to buy up devalued assets from troubled financial firms.
The bailout proposed by the Bush administration is aimed at helping unlock credit and stabilize badly shaken markets in the United States and around the globe.
In the past two weeks, the government has taken over Fannie Mae and Freddie Mac, the country's two biggest mortgage companies, with a bailout plan that could require the Treasury Department to put up as much as $100 billion for each of them over time if needed to keep them afloat as mortgage losses mount.
Last week, the Federal Reserve provided an emergency $85 billion loan to AIG, which teetered on the brink of bankruptcy. Lehman Brothers was forced to file for bankruptcy after attempts to engineer a private rescue fell apart. All the companies were laid low from bad bets on complex mortgage-related securities.
Treasury Secretary Henry Paulson and Federal Reserve Chairman Ben Bernanke made the joint decision last week that the only way to stop the carnage was to deal with the root cause of all the troubles, billions of dollars of bad mortgage debt sitting on the books of major financial companies. This debt has triggered the worst credit crisis in decades, causing credit markets to essentially freeze up despite the fact that the Fed joined with major central banks around the world to pump billions of dollars of reserves into the financial system.
Additionally, the FBI is investigating failed bank IndyMac Bancorp Inc. for possible fraud. Countrywide Financial Corp., formerly the nation's largest mortgage lender and now owned by Bank of America Corp., is also under scrutiny.