It was the second deadly attack in two days on the city that lies just 400 miles (650 kilometers) from the site of the 2014 Winter Olympics. Russian authorities said they believe the latest attack was the work of the same group that set off a deadly bomb Sunday at Volgograd's main railway station.
Together, at least 31 people were killed in the two explosions, putting the city of 1 million on edge and highlighting the terrorist threat that Russia faces as it prepares to host February's Winter Games in Sochi. While terrorists may find it hard to get into tightly guarded Olympic facilities, the bombings have shown they can hit civilian targets elsewhere in Russia with shocking ease.
The heightened security comes as Russians are preparing to celebrate the New Year, the nation's main holiday. In St. Petersburg, Russia's second-largest city, the local governor canceled a New Year's fireworks show.
President Vladimir Putin summoned officials to report on the attacks and sent Alexander Bortnikov, the head of the Federal Security Service, the main KGB successor agency, to Volgograd to oversee the probe. The Sochi Olympics are Putin's pet project.
After meeting with security officials in Volgograd, Bortnikov voiced confidence that officials will quickly find who was responsible for the attacks.
Volgograd, 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 insurgents 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.
"That confirms the investigators' version that the two terror attacks were linked," Markin said in a statement.
Markin said a suicide bomber was responsible for the bus explosion, reversing an earlier statement that the blast was caused by a bomb left behind. At least 14 people were killed Monday and nearly 30 were wounded, according to public health officials. It was not clear if the dead included the bomber.
Seventeen people died in Sunday's suicide bombing, including the bomber, authorities said.
No one has claimed responsibility for either bombing, but they came several months after Chechen rebel leader Doku Umarov threatened new attacks against civilian targets in Russia, including the Olympics.
Volgograd, formerly called Stalingrad, also serves as an important symbol of Russian pride because of the historic World War II battle in which the Soviets turned the tide against the Nazis.
"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, wrote on the organization's website.
Monday's explosion ripped away much of the bus's exterior and shattered windows in nearby buildings. It paralyzed public transport in the city, forcing many residents to walk long distances to get to work.
Police quickly dispersed a few dozen people who attempted to hold an unsanctioned ceremony to commemorate the victims.
Russian authorities have been slow to introduce stringent security checks on bus routes, making them the transport of choice for terrorists. A few months ago authorities began requiring intercity bus passengers to produce identification when buying tickets, like rail or air passengers, but procedures have remained lax.
But even tight rail security is sometimes not enough. On Sunday, the suicide bomber at Volgograd's train station blew up his device in front of the station's metal detectors when a policeman became suspicious. That policeman died and other police were among the some 40 people wounded.
The regional government has introduced five-day mourning for the victims, and nationwide TV stations said they would revise their programming to make it more solemn.
The Interfax news agency quoted an unidentified law enforcement source as saying that a Slavic resident of a Volga River province could have been the railway suicide bomber. It said the man joined the Islamic rebels in Dagestan in 2012 and took an Arabic nom-de-guerre. There was no confirmation of that report from any official sources.
Russia in past years has seen a series of terror attacks on buses, trains and airplanes, some carried out by suicide bombers.
Twin bombings on the Moscow subway in March 2010 by female suicide bombers killed 40 people and wounded more than 120. In January 2011, a male suicide bomber struck Moscow's Domodedovo Airport, killing 37 people and injuring more than 180.
Umarov, who had 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 men to "do their utmost to derail" the Sochi Olympics, which he described as "satanic dances on the bones of our ancestors."
The International Olympics Committee expressed its condolences over Sunday's bombing in Volgograd, but said 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."
Russian authorities have introduced some of the most extensive identity checks and sweeping security measures ever seen at an international sports event.
The security zone created around Sochi stretches approximately 100 kilometers (60 miles) along the Black Sea coast and up to 40 kilometers (25 miles) inland. Russian forces include special troops to patrol the forested mountains flanking the resort, drones to keep constant watch over Olympic facilities and speed boats to patrol the coast.
Anyone wanting to attend the games that open on Feb. 7 will have to buy a ticket online from the organizers and obtain a "spectator pass" for access. Doing so will require providing passport details and contacts that will allow the authorities to screen all visitors.
The security plan includes a ban on cars from outside the zone a month before the games begin.
Associated Press writer Jim Heintz contributed to this report.
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