You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
Financial_crisis_of_2007%E2%80%9308

Context:
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.

Question:
Economist Paul Krugman explained the credit crisis via the implosion of which system?

shadow banking system