A former top U.S. official in charge of investigating the financial crisis said the government has concluded that many inquiries of wrongdoing by financial executives can't succeed as criminal prosecutions.

"There's been a realization and a more deliberate targeting by the Department of Justice before we launch criminally on some of these cases'' said David Cardona, who was a deputy assistant director at the Federal Bureau of Investigation until he left last month for a job at the Securities and Exchange...