CHARLOTTE, North Carolina — American International Group Inc. may have agreed to take the US government up on a two-year, $85 billion loan to help stave off bankruptcy, but now the largest US insurer faces an FBI investigation.

The news, along with concerns that AIG may have to sell all its operating units to pay off the debt, sent AIG shares down more than 20 percent yesterday afternoon.

Law enforcement officials said late on Tuesday that the FBI was investigating the New York-based insurer for potential fraud, as well as mortgage finance companies Fannie Mae and Freddie Mac, and investment bank Lehman Brothers Holdings Inc.

The inquiries will focus on the financial institutions and the individuals who ran them, a 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.

The four financial institutions' recent travails helped trigger the government's $700 billion bailout plan, which continued to be discussed on Capitol Hill yesterday. Lehman Brothers filed for bankruptcy and the government has already taken over Fannie Mae and Freddie Mac.

AIG spokesman Joseph Norton said yesterday the company did not have details on the FBI investigation, but said "of course we will cooperate".

All four companies saw their stock prices plummet this year, as they struggled to survive under the weight of mounting losses tied to bad bets on complex mortgage-related securities.

AIG shares, which lost $1.04, or 20.8 percent, to $3.96 in afternoon trading yesterday, traded as high as $70.13 last October, at the beginning of the credit crisis.

Late on Tuesday, AIG said it signed a definitive agreement with the Federal Reserve Bank of New York for the deal, which was hammered out last week.

A final agreement could be filed as early by end of the week, Norton said.

The agreement provides a two-year, $85 billion emergency loan at an interest rate of about 11.5 percent to AIG, which teetered on the edge of failure because of stresses caused by the collapse of the subprime mortgage market and the credit crunch that ensued.

In return, the government will get a 79.9 percent stake in AIG.

The agreement leaves "AIG essentially nationalised," Bijan Moazami, an analyst at Friedman, Billings, Ramsey, wrote in a note to investors on Wednesday. "Shareholder efforts to prevent the government from taking an equity stake in AIG will prove fruitless."

Some of AIG's shareholders had wanted to help the company raise enough money to avoid taking the loan and ceding a majority stake in the company.

A spokesman for AIG's largest individual shareholder, former Chief executive Maurice "Hank" Greenberg, said yesterday that Greenberg supported those efforts, but declined to comment further.

AIG said on Tuesday it will repay the government loan in full with proceeds from the sales of some of its assets. It will be up to the company to decide which assets to sell and the timing. The government does, however, have veto power.

Shortly after AIG struck the deal, it announced former Allstate Corp. chief executive officer Edward Liddy was taking over as chairman and chief executive. Liddy replaced Robert Willumstad, who took over the company in June.

"AIG made an exhaustive effort to address its liquidity needs through private sector financing, but was unable to do so in the current environment," Liddy said in a statement on Tuesday. "This facility was the company's best alternative. We are pleased to have finalised the terms of the facility, and are already developing a plan to sell assets, repay the facility and emerge as a smaller but profitable company."

He said AIG's insurance subsidiaries remain "strong, liquid and well-capitalised".