VOLGOGRAD, Russia -- Another deadly blast has struck the southern Russian city of Volgograd, killing at least 14 people and further highlighting Russia's security challenges as it readies to host the Winter Olympics in less than six weeks.
An explosion hit a trolleybus near a busy market during the morning rush hour Monday, a day after a blast at Volgograd's main train station killed 17 people and wounded at least 35 others.
Like Sunday's attack, the blast Monday was a terrorist act, Vladmir Markin, a spokesman for the country's federal investigation agency, told the state-run news agency RIA Novosti.
No one claimed responsibility for the explosions. But they come several months after the leader of a Chechen separatist group pledged violence to disrupt the 2014 Sochi Winter Olympics.
Two blasts in two days
Video footage from the scene Monday showed the twisted shell of the blue trolleybus, with debris spread around it. The impact of the blast blew out the roof of the bus, as well as windows of several nearby houses. At least 28 people were reported to be wounded. Several are in serious condition, including a 6-month-old baby, RIA Novosti reported.
Based on the footage, the blast appeared to have occurred in the back half of the bus. The federal investigation agency said it believes the explosion was set off by a male suicide bomber.
And there are strong indications the two attacks are linked, the agency said.
Investigators said the train station blast Sunday appeared to have been caused by a suicide bomber, who may have been female.
Markin told RIA Novosti that DNA testing will be carried out on the remains of the station bomber, who used the equivalent of 10 kilograms (22 pounds) of TNT in a device containing shrapnel. Investigators say they also found an unexploded grenade at the scene.
Video taken from an outside security camera shows a huge fireball inside what appears to be the main entrance of the three-story stone building, followed by a steady trail of smoke coming out of shattered windows.
The approaching Olympics
The deadly explosions come ahead of the Winter Olympics in Sochi, which is less than 1,000 kilometers (620 miles) southwest of Volgograd. The Games will take place between February 7 and 24.
Once called Stalingrad, Volgograd is a major rail hub in the region, and each day thousands of passengers pass through the station, many on their way to Moscow.
In October, a bomber blew up a passenger bus in Volgograd, killing six people and wounding more than 30 others. Russian media reported that a female Islamist suicide bomber from the Russian region of Dagestan was responsible for that attack.
"Most of the militants responsible for terrorist attacks in Russia over the last decade -- including female suicide bombers who have taken part in 20 attacks claiming at least 780 lives since June 2000 -- have come from Dagestan," RIA Novosti reported Monday.
The two young men behind the Boston Marathon bombings lived briefly in Dagestan before coming to the United States, and one of them visited the area the year before the attack.
"Radical Islamist groups tied to the war against the Russian state now have much deeper roots, and are far more active, in Dagestan than in Chechnya," Rajan Menon, senior fellow at the Atlantic Council, wrote in a CNN.com column.
Russia's security challenge
Russian President Vladimir Putin has maintained that the Sochi games will be safe and security will be tight.
Visitors to Sochi and the surrounding area are subjected to rigorous security checks, and vehicle license plates are monitored.
But the deadly blasts in Volgograd highlight the challenge that Russian authorities face in policing the rest of the country amid ongoing unrest in the North Caucasus. That region includes Chechnya, where Russia fought two wars against separatist movements, and Dagestan.
Volgograd is a hub for people traveling to Sochi from Moscow and other parts of central Russia. It also has key transportation links with the North Caucasus.
In July, Doku Umarov, the leader of the Chechen group Caucasus Emirate, released a video statement in which he vowed to unleash "maximum force" to disrupt the games at Sochi.
The U.S. State Department considers the Caucasus Emirate a foreign terrorist group and has authorized a reward of up to $5 million for information leading to the location of Umarov.
The State Department said Umarov organized a suicide bombing outside the Chechen Interior Ministry in May 2009.
His group also claimed responsibility for the 2011 bombing of Domodedovo Airport in Moscow that killed 36 people, the 2010 bombings of the Moscow subway that killed 40 and the 2009 bombing of the high-speed Nevsky Express train in which 28 people died.
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