Rubin said financial risks were too small to concern board

During his stint at Citigroup, former Treasury Secretary Robert Rubin said models showed negligible risks for holding sub-prime investments and the instruments didn't merit attention of the bank's board. 

He said one model showed a 1-in-10,000 chance of the investments turning toxic. 

"In hindsight, there were obviously real problems," he said. 

Rubin is testifying with former Citigroup CEO Chuck Prince before the Financial Crisis Inquiry Commission, a group charged with examining the financial crisis and report its findings to Congress by Dec. 15.