Mr. Powell has emphasized the importance of oversight by bank boards of directors, and the Wells action criticized the company’s board for failing to do its job. The Fed also published harshly critical letters to the last two chairmen of the Wells board, holding them accountable for the problems.

Mr. Powell is a lifelong Washingtonian. He went away to college at Princeton but came back to law school at Georgetown. He went away to work on Wall Street but came back in the early 1990s to take a job under Treasury Secretary Nicholas Brady, who had also been his boss at the white-shoe investment bank Dillon Read.

Mr. Powell has the calm and unassuming manner of a person confident in his abilities. Mr. Carpenter said the only time that he had seen Mr. Powell show irritation was when he persisted in addressing Mr. Powell as “Governor Powell” rather than “Jay.”

John Dugan, a longtime friend who worked for Mr. Powell at Treasury, said Mr. Powell’s success owed much to playing well with others. “He has a tremendous sense of humor, but not an unkind one,” said Mr. Dugan, a former comptroller of the currency.

The Salomon scandal began with a routine phone call. At the time, Wall Street firms wrote bids on scraps of paper stuffed into a wooden box. The New York Fed policed the process by calling a small selection of buyers to verify bids. In the spring of 1991, a staff member reached a surprised supposed bidder. A quiet investigation found that Salomon was bidding in other peoples’ names. No firm was allowed to buy more than 35 percent of a given debt issue; at one auction in May 1991, Salomon bought 94 percent, allowing the firm to resell at a premium.

Mr. Powell oversaw the government’s borrowing program, so he led the repairs, including the negotiations over a long weekend in August that resulted in the departure of Salomon’s top management and the installation of Warren E. Buffett as chairman. The bond trader went to prison.

Mr. Powell “was impressive to me because he thought clearly and he didn’t jump to conclusions,” said Deborah A. Perelmuter, then the head of the domestic trading desk at the New York Fed, who watched Mr. Powell hammer together a consensus on the necessary changes — including the replacement of paper slips with electronic bidding. “He seemed to have his head on straight.”