text: Economist Paul Krugman and U.S. Treasury Secretary Timothy Geithner explain the credit crisis via the implosion of the shadow banking system, which had grown to nearly equal the importance of the traditional commercial banking sector as described above. Without the ability to obtain investor funds in exchange for most types of mortgage-backed securities or asset-backed commercial paper, investment banks and other entities in the shadow banking system could not provide funds to mortgage firms and other corporations.