Washington ? WorldCom executives clashed with former auditors Monday over responsibility for nearly $4 billion in accounting improprieties that rocked U.S. markets. The telecommunications giant’s former CEO and finance chief refused to testify to a House panel investigating the debacle.

WorldCom Chairman Bert Roberts called auditor Arthur Andersen’s failure to uncover the irregularities “inconceivable.”

Former Andersen partner Melvin Dick countered that auditors rely “on the honesty and integrity of the management of the company.” He said he understands WorldCom’s former chief financial officer, Scott Sullivan, has acknowledged he never told the accounting firm about the questionable bookkeeping.

Sullivan invoked his Fifth Amendment right against self-incrimination before a packed hearing of the House Financial Services Committee, saying he was doing so “based upon the advice of counsel.”

WorldCom’s former chief executive officer, Bernard Ebbers, did the same, saying his Washington attorney, Reid Weingarten, advised him to remain silent because of ongoing investigations by the Justice Department and the Securities and Exchange Commission.

“I do not believe I have anything to hide,” Ebbers said. When all the facts are out, he said, “I believe that no one will conclude that I engaged in any criminal or fraudulent conduct.”

Members of the panel Democrats and Republicans alike attacked the company, the Andersen accounting firm and Wall Street analyst Jack Grubman, who promoted WorldCom stock.

Several grilled company founder Ebbers, asking for example about the $400 million in loans he received from WorldCom and his $1.5 million severance package. Ebbers, sitting stonily with arms crossed, repeatedly cited his Fifth Amendment privilege and said he would not answer.

WorldCom is the latest corporation to face allegations of executive wrongdoing and accounting irregularities driving down confidence in business and the stock market.