An ambulance leaves the site of a trolleybus explosion in Volgograd, Russia, Monday, Dec. 30, 2013. The explosion left 10 people dead Monday, a day after a suicide bombing that killed at least 17 at the city's main railway.MOSCOW - An explosion on a trolleybus in the city of Volgograd left 10 people dead Monday, 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.
The National Anti-Terrorism Committee said the bus explosion was most likely caused by a bomb, but there were no further details.
There was no immediate claim of responsibility for either explosion, which came several months after Chechen rebel leader Doku Umarov called for new attacks against civilian targets in Russia, including the Sochi Games.
Suicide bombings have rocked Russia for years, but most have been in the North Caucasus region, the center of an insurgency seeking an Islamist state in the region. But Volgograd, formerly known as Stalingrad, has been struck three times in two months - suggesting militants may be using the transportation hub as a renewed way of showing their reach outside their restive region.
A suicide bus bombing in Volgograd in October killed six people.
In the railroad station blast, the bomber detonated explosives in front of a metal detector just beyond the station's main entrance when a police sergeant became suspicious and rushed forward to check ID, officials said. The officer was killed by the blast, and several other policemen were wounded.