Federal investigators have opened preliminary probes of four high-profile companies at the center of the financial turmoil.

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

Neither Lehman and AIG would nor discuss the inquiries. A spokeswoman for the Federal Housing Finance Agency, which regulates Fannie and Freddie and has both government-sponsored enterprises in conservatorship, also declined to comment.

The huge potential tab for taxpayers raises the stakes in the crisis for the Justice Department.

The FBI says it has 26 companies under investigation, in addition to pursuing more than 1,400 mortgage fraud cases nationwide.

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 FBI said.

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