In early 2008, he became a top executive in the Citigroup unit that housed many of the bank’s riskiest operations, including its hedge funds and private equity investments. Massive losses in that unit helped drive Citigroup into the arms of the federal government, which bailed out the bank with $45 billion in taxpayer money that year.

The group had been under pressure to compete with similar units at other big Wall Street firms and, some analysts say, took on too many risks as it played catch-up.

“The mismanagement of risk was comprehensive at that organization,” said Simon Johnson, an economist at the Massachusetts Institute of Technology.

Details about Lew’s exact responsibilities at Citigroup, where he worked from 2006 to 2008, are scant. He declined to comment for this article.