The FBI is investigating Lehman Brothers and three other contributors to America’s financial crisis to determine whether they put pressure on ratings agencies to award top ratings to securities they issued.

Concerns that Fannie Mae, Freddie Mac, AIG or Lehman may have sought to encourage agencies to inflate their ratings — by offering higher fees or the promise of more work — form part of a broad inquiry by the bureau.

The agencies are widely regarded as having failed debtholders by attributing the top ratings to many securities that turned out to be extremely risky and have lost investors hundreds of billions of dollars.

The FBI, which is also investigating whether any of the four institutions deliberately misled investors about the true health of their assets, is expected to demand that they “hold all papers and e-mails under lock and key” as it sifts through the evidence, a source said.

Investigators are expected to begin their work in the accounts departments of the four companies with the intention of seeing where a trail might lead. The FBI believes that the near-collapse of the four triggered the Government’s proposed $700 billion (£378 billion) bailout of Wall Street by putting the entire financial system in jeopardy.

Although the bureau intends to bring to justice any company or individual found guilty of wrongdoing, one source said that the investigations were at a preliminary stage only and that no evidence of fraud had been uncovered so far.

The FBI, which is investigating 24 cases of possible corporate fraud in relation to the worst housing crisis since the Great Depression, has been under pressure to use its powers to bring the architects of any related fraud to justice. That pressure has become more intense after the announcement that $700 billion of taxpayers’ funds could be used to prop up the financial services groups that many think created the crisis through reckless lending. This bailout would be in addition to the near-$300 billion that the US Government has pledged to Fannie Mae, Freddie Mac and AIG.

Patrick Leahy, the Vermont senator who is chairman of the Senate Judiciary Committee, told Robert Mueller, the FBI Director, at a hearing last week: “The US 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.”

The FBI declined to comment on the latest cases, as did the four companies. However, a spokesman for the bureau said: “The FBI continues to investigate a number of companies for sub-prime 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.”

Shares in Fannie Mae rose 32 per cent by the close and Freddie Mac’s by 42 per cent as brokers continued to unwind trades after Friday’s ban on short selling in the stocks of many American financial groups. Lehman’s shares closed 21 per cent higher while AIG tumbled by 25 per cent.

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