Victoria Harman, a spokesman for CSFB, would not confirm or deny the firm had been issued a subpoena in connection with the probe. She added that, "CSFB simply handled a customer's trade in AIG shares. We are cooperating fully with the authorities in this matter."

The investigation of AIG deals with its behavior during the period when it was about to acquire Houston-based American General in a deal for $23 billion in AIG stock. Completion of the acquisition would have been more costly for AIG if its stock price fell below a certain threshold in a 10-day period ending three days before the deal closed.

The Wall Street Journal article said that CSFB is not a target of the probe, but has received a subpoena for information about its AIG trading activities during the period in question.

But, AIG categorically denied wrongdoing. Joe Norton, a spokesman, said AIG "was not buying stock during the pricing period nor did anyone to our knowledge buy AIG stock on AIG's behalf. These anonymous allegations are unfounded and irresponsible."

The Journal has previously reported that investigators were probing whether AIG Chairman Maurice Greenberg brought pressure on the specialist firm that oversees AIG stock trading.