There was a time, after the financial crisis, when federal agents and prosecutors thought they had another savings-and-loan type situation on their hands. Those criminal probes in the 1980s and 1990s landed more than 1,000 bankers in prison.

But their hopes slowly gave way to frustration over how to prove criminal intent, according to David Cardona, who was a deputy assistant director at FBI until he left last month for a job at the SEC.