Former US Secretary of State Mr Henry Kissinger has stepped down as chairman of a panel investigating the September 11th terrorist attacks on New York and Washington.
He cited controversy over potential conflicts of interest with his private-sector clients as the reason.
"It is clear that, although specific potential conflicts can be resolved in this manner, the controversy would quickly move to the consulting firm I have built and own," said Mr Kissinger.
In a letter to George W Bush, who appointed him, he added: "I have, therefore, concluded that I cannot accept the responsibility you proposed."
The decision was another blow for the fledging panel and the families of September 11 victims. The panel's original vice chairman, Mr George Mitchell, resigned from the commission on Wednesday, partly because of pressures to quit his law firm.
Mr Kissinger's resignation came a day after he tried to assure victims that his business interests would not conflict with his duties as chairman.
The White House and congressional Democrats had clashed on whether he had to disclose his business clients, with Mr Bush's advisers saying the law did not require such disclosures.
Mr Bush issued a written statement saying he accepted Mr Kissinger's resignation with regret.
"His chairmanship would have provided the insights and analysis the government needs to understand the methods of our enemies and the nature of the threats we face," the statement said.
He promised to pick a new chairman to help "uncover every detail and learn every lesson of September 11th, even as we act on what we have learned so far to better protect and defend America".
PA