(Fortune) -- Two former CEOs of AIG tried to make the case at a Congressional hearing Tuesday that the downfall of the giant insurer and financial services firm wasn't their fault, but rather that of external forces, including an accounting rule.

Members of the House Committee on Oversight and Government Reform weren't buying it, not that lawmakers were necessarily well-versed in the rule in question, known as mark-to-market. But they seemed to know a half truth when they heard one.

Former chief executives Martin Sullivan, who resigned under pressure in June andRobert Willumstad, a former president of Citigroup who headed AIG until the government bailed out the company last month with an $85 billion loan, attributed the bulk of the insurer's problems to forces beyond their control.

Both men cited the unintended consequences of mark-to-market accounting, a practice that forced AIG to book unrealized losses on its book of mortgage-backed securities and credit default swaps - a type of contract which can insure everything from plain-vanilla corporate and municipal debt to esoteric products like collateralized debt obligations, or CDOs. AIG was a big player in both the default swap and mortgage-backed debt markets.