WASHINGTON — Federal investigators have opened preliminary probes into the financial troubles of four high-profile companies that are at the center of the current financial turmoil that the Bush administration says requires an unprecedented proposed taxpayer-funded bailout to clean up.

The Federal Bureau of Investigation's preliminary inquiries are focusing on whether fraud helped cause some of the troubles at Fannie Mae, Freddie Mac, Lehman Brothers Holdings Inc. and American International Group Inc., according to senior law-enforcement officials.

Lehman and AIG declined to comment. A spokeswoman for the Federal Housing Finance Agency, which regulates Fannie and Freddie and has both companies in conservatorship, also declined to comment.

The probes come as the huge potential tab for taxpayers in the crisis raises the stakes for the Justice Department. The FBI says it now has 26 companies under investigation, in addition to pursuing more than 1,400 mortgage-fraud cases nationwide.

Pressure is building for the FBI and regulators to hold top executives accountable for the crisis that has crippled the nation's finance sector. In meetings on Capitol Hill, some lawmakers raised concerns with Treasury Secretary Henry Paulson that by taking large stakes in some financial firms, the government may be limiting its ability to exact penalties for wrongdoing, according to people familiar with the matter.

The FBI says that the corporate probes are part of an effort to pursue allegations of higher-level fraud more sweeping than the retail-level infractions that have been at the center of most cases brought so far.

The U.S. Attorney in Brooklyn has brought charges against brokers who allegedly tricked some institutional investors into buying risky auction-rate securities.

During the Savings and Loan bailout 20 years ago, federal prosecutors brought more than 600 cases against 1,000 defendants. But the complicated securities at the heart of today's crisis make comparable prosecutions more difficult, investigators say.

Brian Roehrkasse, Justice Department spokesman, defended the way the department has responded to the crisis: "The FBI continues to investigate a number of companies for subprime-lending practices, but the department brings criminal prosecutions based solely on the facts and the law. Where we find evidence of criminal wrongdoing, we will prosecute."

Investigators say that despite calls from some quarters to prosecute wealthy bankers who helped fuel the mortgage bubble, it is unclear what crimes they will find at the root of the exotic financial vehicles that have sickened banks around the world. A more likely outcome of the probes will be hundreds more retail-level fraud cases of the type already being brought against brokers, real-estate agents, and buyers related to falsified mortgage applications.

There is yet to emerge a figure such as banker Charles Keating, who served four years in prison for fraud related to the collapse of American Financial Corp. and whose name became synonymous with the S&L crisis. But already there is widespread anger that mortgage securities deals enriched many on Wall Street at the expense of millions of home buyers and taxpayers nationwide who will end up paying the costs.

Democratic Sen. Patrick Leahy of Vermont, chairman of the Senate Judiciary Committee, told FBI Director Robert Mueller at a hearing last week, "The U.S. government is on the hook [for] anywhere from $800 billion to $1 trillion. And if people were cooking the books, manipulating, doing things they were not supposed to do, then I want people held responsible."

Some in Congress have gone further, accusing the FBI of warning of the impending mortgage-fraud epidemic but not doing enough to stop it. FBI officials say their focus on mortgage fraud has produced 3,500 cases and more than 700 convictions since 2005, but that the collapse of the housing market was rooted more in risky business practices than in fraud.

Comparisons with the S&L crisis are a mismatch for several reasons, prosecutors say. In the case of the S&L bailout, the institutions were federally regulated, which made it easier for prosecutors to pursue charges based on improper disclosures. And when the government's Resolution Trust Corporation took over a thrift, regulators put in place a system to turn over information that could assist investigators looking for criminal wrongdoing.

Investigators and lawyers say today's crisis differs in that, at the height of the mortgage-fueled boom, more than 50% of subprime mortgages originated with private mortgage providers that had no federal supervision. That means investigators trying to prove investors were misled have to dig deeper into how lenders were doing business. Thrifts were regulated institutions, and many of the criminal cases brought focused on failure to file required documentation.

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