WASHINGTON—Former Citigroup Inc. Chief Executive Charles Prince expressed shame and contrition during a three-hour grilling by a congressional panel scrutinizing the financial crisis. But Robert Rubin, the former U.S. Treasury secretary who led Citigroup's executive committee, defiantly defended his role in decisions that led to gigantic losses at the bank.

Thursday's hearing by the Financial Crisis Inquiry Commission broke the recent silence by the two men about their culpability for more than $58 billion in write-downs by Citigroup on assets tied to risky subprime mortgages and other financial instruments. Mr. Prince has said little about Citi's problems since resigning under pressure in November 2007, while Mr. Rubin has been mostly quiet since announcing his retirement early last year.

Mr. Rubin reiterated his previous defense that his role was peripheral to the New York company's main operations even though he was one of its highest-paid officials. He noted repeatedly during the testy hearing that he didn't want to hold the same kind of "significant operational responsibility" at Citigroup that he had as Treasury secretary or while working at Goldman Sachs Group Inc.

Even his lofty title at Citigroup didn't involve frequent meetings and "wasn't a substantive part of the decision-making process," Mr. Rubin told the panel.

In contrast, Mr. Prince departed from his prepared testimony by saying "I'm sorry" three times that the crisis had such a "devastating" impact on the economy, millions of Americans "lost their homes," and "our management team, starting with me," didn't foresee the "unprecedented market collapse."