Investigators work at the site of an explosion at the entrance to a train station in Volgograd December 29, 2013. A female suicide bomber blew herself up in the entrance hall of the Russian train station on Sunday, killing at least 13 people in the second deadly attack in the space of three days as the country prepares to host the Winter Olympics. REUTERS/Sergei Karpov (RUSSIA – Tags: CRIME LAW CIVIL UNREST TRANSPORT TPX IMAGES OF THE DAY)