Sherry Hunt started processing home loans at a small Alaska bank in 1975. She enjoyed the work and rose through the ranks in the mortgage loan business, ultimately becoming a vice president at Citigroup‘s (NYSE:C) mortgage fraud unit in 2004, advising against risky loans.

This year, as a whistle-blower against Citigroup, Hunt received $31 million under a settlement between the bank and federal regulators over improperly approved mortgages, Bloomberg reports.

At Citigroup, Hunt’s unit verified loan paper work, borrower income and the appraised values of homes. Citigroup could then guarantee the quality to the mortgages when they were later sold to the government or private investors.

The bank purchased so many mortgages from external lenders that by 2006, it was impossible to check every loan. Hunt warned her superiors that mortgages with shoddy paperwork and without proper signatures were being approved.

By 2007, Hunt says that as many as 60% of loans bought and sold by Citigroup lacked some element of required documentation.

Even after the financial crisis, Hunt says that Citibank continued to approve unverified loans. Hunt started writing down how the bank evaded mortgage rules. In 2011, a superior told her that she needed to lower the number of loans declared defective or face consequences.

Hunt notified the bank about the problems in its mortgage fraud unit, then she called federal regulators. Three months later, she sued Citigroup for violating U.S. mortgage regulations in a New York court. The federal government joined her suit in January and Citigroup settled with the government for $158.3 million in February.