Former AIG chief executive Maurice 'Hank' Greenberg has resigned from its board.

Greenberg wrote to AIG expressing frustration at being blocked from accessing company documents. AIG's board refused access as Greenberg had been named in allegations of the mis-stating of company accounts.

The Justice Department has continued to build a case against Greenberg.

Meanwhile, this week a second General Re executive was charged with conspiracy to falsify financial filings.

Richard Napier, who is cooperating with inquiries, pleaded guilty to the charges. The Justice Department said executives "knew that high level executives, including the chief executives of Gen Re and AIG [Greenberg] instigated a sham transaction in which he participated."

It is alleged that AIG books were bolstered by up to $500m by the "sham" transactions.