This in turn, said former AIG chief executive Willumstad, who ran the company for just three months after Sullivan left, forced the firm to raise billions of dollars in capital. The federal rescue came after AIG suffered disastrous liquidity problems after its credit rating was lowered, forcing the company to come up with even more capital.

"AIG was caught in a vicious cycle," Willumstad said in the testimony.

Greenberg said that AIG "wrote as many credit default swaps ... in the nine months following my departure as it had written in the entire previous seven years combined. Moreover, "unlike what had been true during my tenure, the majority of the credit default swaps that AIGFP wrote in the nine months after I retired were reportedly exposed to subprime mortgages."

But Sullivan said the complex swaps had underlying value, even as the market for them froze, sending their book value plummeting and forcing AIG to scramble for collateral.

"When the credit markets seized up, like many other financial institutions, we were forced to mark our swap positions at fire-sale prices as if we owned the underlying bonds, even though we believed that our swap positions had value if held to maturity," Sullivan said.

The hearing is the second in two days into financial excesses and regulatory mistakes that have spooked stock and credit markets and heightened fears about a global recession.