bernanke further explained that specific triggering events began in mid-2007, as investors began to withdraw funds from the shadow banking system, analogous to depositors withdrawing money from depository banks in past bank runs. investors became unsure of the value of the securities (loan collateral) held by the shadow banks, as many derived their value from subprime mortgages. mortgage companies could no longer borrow money to originate mortgages, and many failed in 2007. the crisis accelerated in 2008, as the largest five u.s. investment banks, which had $4 trillion in liabilities by the end of 2007, could no longer obtain financing. they had grown increasingly dependent on short-term sources of financing (e.g., repurchase agreements), and were unable to obtain new funding from investors. these investment banks were forced to sell long-term securities at fire-sale prices to meet their daily financing needs, suffering enormous losses. concerns about the possible failure of these banks led the financial system to essentially freeze by september 2008. the federal reserve increasingly intervened in its role as lender of last resort to stabilize the financial system as the crisis deepened.