European stock indices suffered from a significant sell-off of bank shares today, Friday, as concerns about the stability of the financial sector increased. Deutsche Bank's shares fell amid a surge in the cost of insurance on the German bank's debts against default risks, reaching the highest level in over four years. The Stoxx 600 index sank 1.4%, but still recorded a weekly gain supported by a strong recovery earlier in the week. Deutsche Bank's stock plummeted by 8.5% following a steep rise in the cost of default risk insurance. Shares of UBS and Credit Suisse dropped by 3.6% and 5.2% respectively after Bloomberg reported that they are among the banks being examined in a U.S. Justice Department investigation into whether financial services specialists helped Russian businessmen evade sanctions. The European banking index fell by 3.8% and marked its third consecutive weekly decline after the collapse of mid-sized American banks and turmoil at Credit Suisse highlighted the growing risks facing banks following tightened monetary policy.