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A suicide bomber killed at least 14 people and wounded 28 aboard an electric bus in the southern Russian city of Volgograd during the Monday morning rush hour, and authorities believe it was the work of the same group that set off a bomb at a railway station a day earlier.
While attackers may find it hard to get to the tightly guarded Olympic facilities, the bombings have shown that rebels can hit civilian targets elsewhere in Russia with shocking ease.
Volgograd, located about 425 miles northeast of Sochi, serves as a key transport hub for southern Russia, with numerous bus routes linking it to volatile provinces in Russia's North Caucasus, where rebels have been seeking an Islamic state.
Vladimir Markin, the spokesman for Russia's main investigative agency, said Monday's explosion involved a bomb similar to the one used in Sunday's attack at the city's main railway station, in which at least 17 people were killed and scores more were injured.
"That confirms the investigators' version that the two terror attacks were linked," Markin said in a statement. "They could have been prepared in one place."
Markin said a suicide attacker was responsible for the Monday bus explosion — a change from an earlier official statement that the blast was caused by a bomb left in the vehicle's passenger area.
No one has claimed responsibility for either bombing, but they come several months after Chechen rebel leader Doku Umarov threatened new attacks against civilian targets in Russia, including the Olympics in Sochi.
The nation's Foreign Ministry said in a statement that the attacks were an "attempt by terrorists to open an internal front," and likened the two explosions to attacks in the United States, Syria and elsewhere.
"We will not retreat," the statement said.
The United Nations Security Council condemned the attack "in the strongest terms" Monday.
"The members of the Security Council reaffirmed that terrorism in all its forms and manifestations constitutes one of the most serious threats to international peace and security," the 15-member council said in a press statement.
Suicide bombings and other terrorist attacks have rocked Russia for years but most recently had been confined to the North Caucasus region. The successive attacks in Volgograd signaled that rebels may be using the transportation hub as a renewed way of showing their reach outside their restive region.
Volgograd, when it was named Stalingrad, was the site of one the most important battles of World War II, in which the Soviets stopped the German advance into the U.S.S.R. The battle holds an important place in the Russian psyche and increases its symbolic value to attackers.
"Volgograd, a symbol of Russia's suffering and victory in World War II, has been singled out by the terrorist leaders precisely because of its status in people's minds," Dmitry Trenin, the head of the Carnegie Endowment's Moscow office, said in a commentary posted on the organization's website.
Because of recent violence, security remains a concern throughout southern Russia before the Olympics in the Black Sea resort of Sochi, which is near the restless North Caucasus.
Monday's explosion ripped away much of the bus's exterior and shattered windows in nearby buildings. It virtually paralyzed public transport in the city, forcing many residents to walk long distances to get to work.
Russian authorities have been slow to introduce stringent security checks on bus routes, making them the transport of choice for rebels in the region. A few months ago, authorities introduced a requirement for intercity bus passengers to produce identification when buying tickets, like rail and air passengers, but procedures have remained lax, and some of the routes aren't controlled.
Even tight railway security is sometimes not enough. In Sunday's suicide bombing, the attacker detonated explosives in a crowd in front of the station's metal detectors.
A suicide bus bombing in Volgograd in October killed six people. On Friday three people were killed when an explosives-rigged car blew up in the city of Pyatigorsk, the center of a federal administrative district created to oversee Kremlin efforts to stabilize the North Caucasus region.
After the October incident, Russian authorities said they started taking saliva samples from religiously conservative women in the area to identify them if they became suicide bombers.
In Sunday's railroad station blast, the bomber detonated explosives just beyond the station's main entrance when a police sergeant became suspicious and rushed forward, officials said. The officer was killed by the blast, and several other policemen were among some 40 people wounded.
The Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia. Putin on Monday summoned the interior minister and the chief of the Federal Security Service, the main KGB successor agency, to discuss the situation and sent the FSS head to Volgograd to oversee the probe.
Umarov, who claimed responsibility for the 2010 and 2011 bombings, ordered a halt to attacks on civilian targets during the mass street protests against Putin in the winter of 2011–12. He reversed that order in July, urging his followers to "do their utmost to derail" the Sochi Olympics, which he described as "satanic dances on the bones of our ancestors."
The International Olympic Committee expressed its condolences over Sunday's bombing in Volgograd, adding that it was confident of Russia's ability to protect the games.
Russian Olympic Committee chief Alexander Zhukov said Monday there was no need to take any extra steps to secure Sochi in the wake of the Volgograd bombings, as "everything necessary already has been done."