At least 14 people were killed and 28 wounded when an explosion ripped through a trolley bus in the Russian city of Volgograd on Monday, the city's second terror strike in two days. The explosion, which tore out much of the electric vehicle's exterior and left mangled bodies on the street, raised fears about more violence in the lead-up to the Sochi Winter Olympics that Russia will host in six weeks. Volgograd, a city of around one million about 400 miles northeast of Sochi, is a key transport hub for southern Russia. On Sunday, at least 16 people were killed in another attack at a railway station in Volgograd. On Friday, a car bomb killed three people in the southern Russian city of Pyatigorsk, 170 miles east of Sochi. Although no group has claimed responsibility for the attacks, leaders of an insurgency that aims to create an Islamic state out of Muslim provinces south of Volgograd have urged militants to use "maximum force" to stop the Olympics. President Barack Obama has been briefed on the attacks and the United States and Russia were cooperating on anti-terrorism leading up to the Games, the White House said.