Denis Tyrin / AP
An ambulance leaves the site of a trolleybus explosion in Volgograd, Russia, Monday, Dec. 30, 2013. The explosion left 10 people dead, a day after a suicide bombing that killed at least 17 at the city's main railway. The explosions put the city on edge and highlighted the terrorist threat that Russia is facing as it prepares to host the Winter Games in February. Volgograd is about 650 kilometers (400 miles) northeast of Sochi, where the Olympics are to be held.