International investigators hunted for those behind an unprecedented cyber-attack affected systems in dozens of countries. Banks, hospitals and government agencies were worst affected as security experts sought to contain the fallout. <br/><br/>The assault, which began Friday and was being described as the biggest-ever cyber ransom attack, struck state agencies and major companies around the world — from Russian banks and British hospitals to FedEx and European car factories.<br/><br/><br/>Europol said a special task force at its European Cybercrime Centre was specially designed to assist in such investigations and will play an important role in supporting the investigation. The attacks used ransomware that apparently exploited a security flaw in Microsoft operating systems, locking users' files unless they pay the attackers a designated sum in the virtual currency Bitcoin.<br/><br/>Mikko Hypponen, Chief Research Officer at the Helsinki- based cyber security company F-Secure, said that it was the biggest ransomware outbreak in history, saying that 1 lakh 30 thousand systems in more than 100 countries had been affected. <br/><br/>He said Russia and India were hit particularly hard, largely because Microsoft's Windows XP — one of the operating systems most at risk- was still widely used there.