MBIA said it was asked by the SEC and Mr. Spitzer's office for more than four years of records detailing its accounting for various aspects of the insurance policies it sells, as well as its transactions with a Bermuda company it helped establish last year. MBIA owns a 17.4% stake in Bermuda reinsurer Channel Reinsurance Ltd. MBIA officials said the company helped establish Channel Re to gain access to a triple-A-rated reinsurer, and that the policies it buys from Channel Re are traditional reinsurance, not the nontraditional type that are under regulatory scrutiny.

MBIA said the subpoenas also asked about the company's purchase of so-called credit-default protection on itself. The "credit-default swap" market allows investors to buy insurance against defaults by bond issuers. MBIA said it is cooperating with the authorities.

At AIG, meanwhile, executives in the company's corporate-finance department, which oversees the drafting of the company's financial statements, will likely be a focus for investigators, the people said. Criminal charges, if any, are most likely for AIG staffers who intentionally misled New York and Pennsylvania insurance regulators in recent weeks when they asked questions about AIG's accounting for transactions with the two offshore reinsurers with which AIG Wednesday acknowledged Wednesday having previously undisclosed ties, Union Excess Reinsurance Co. and Richmond Insurance Co.

And other accounting issues already detailed by AIG in its Wednesday release -- for instance, including possibly wrongly boosting net investment income, another item closely watched by investors -- have led regulators to wonder whether current AIG executives were aware of these improprieties. Some institutional investors said they have been assured by the company that there are no indications suggesting newly appointed Chief Executive Martin Sullivan will be implicated in any wrongdoing.

Some Wall Street analysts braced for more bad news at AIG. Problems identified so far are primarily in the company's accounting for its U.S. units, where transparency is highest, rather than overseas, where less disclosure is often required, cautioned Alain Karaoglan, an insurance analyst with Deutsche Bank Securities Inc. Profit margins for the company's foreign life-insurance business "have been very strong -- maybe too strong," Mr. Karaoglan wrote.

The AIG probes began with scrutiny last year by the SEC and Mr. Spitzer of a transaction in 2000-01 between AIG and Berkshire Hathaway Inc. that had been initiated by Maurice R. Greenberg, AIG's chief executive for nearly four decades until the mounting scrutiny prompted his retirement two weeks ago. Mr. Greenberg since has said he will leave his post as chairman of the company. Three senior AIG executives, including the company's chief financial officer, have been dismissed after refusing to answer investigators' questions, exercising their Fifth Amendment rights to avoid incriminating themselves.

Write to Ian McDonald at ian.mcdonald@wsj.com¹⁰ and Theo Francis at theo.francis@wsj.com¹¹