WASHINGTON — An inquiry panel is hearing from former CEOs of two big banks that succumbed to the financial crisis, Lehman Brothers and Wachovia, as it delves into the "too big too fail" predicament and potential systemwide risk from financial institutions.

Federal bank regulators also are appearing today before the bipartisan Financial Crisis Inquiry Commission, established by Congress to investigate the financial meltdown that plunged the economy into the most severe recession since the 1930s.

After the subprime mortgage bubble burst in 2007, complex investments called credit default swaps, which insured against default of securities tied to the mortgages, collapsed. That brought the stunning downfall of Lehman Brothers Holdings. Its implosion into the biggest bankruptcy in U.S. history on Sept. 15, 2008, triggered a worldwide panic in financial markets.

U.S. officials, as they scrambled to avert economic catastrophe, declined to rescue the once-venerable Wall Street titan while injecting tens of billions of dollars into others — like the insurance conglomerate American International Group.

Aided and prodded by the government, Wells Fargo & Co. acquired Charlotte, N.C.-based Wachovia, which had done a huge business in adjustable-rate mortgages, enticing borrowers who later defaulted on their home loans. That $12.7 billion deal, announced in early October 2008, created a coast-to-coast powerhouse with operations in 39 states and the District of Columbia.

Scheduled to testify at today's hearing are Wachovia's former President and CEO Robert Steel; former Lehman Chairman and CEO Richard S. Fuld Jr.; Scott Alvarez, general counsel of the Federal Reserve; Thomas Baxter, general counsel and executive vice president of the New York Fed; John Corston, an official of the Federal Deposit Insurance Corp.; and Barry Zubrow, chief risk officer at JPMorgan Chase & Co.

Under the landmark financial overhaul law enacted in July, regulators are empowered to shut down financial institutions whose collapse could threaten the system, ending the doctrine of "too big to fail."

Fuld, a towering figure whose nickname was "Gorilla," has publicly conceded no errors or misjudgments in the chaotic period that led to Lehman's bankruptcy. He told a congressional hearing in October 2008 that the firm did everything it could to limit its risks and save itself. It failed, he said, because of a "crisis in confidence" on Wall Street, market manipulation in which investors preyed on distressed financial players by betting on their demise, and would-be buyers who waited for the government to step in to help fund a sale.