The deaths of at least 16 people in a suicide bombing in southern Russia on Sunday fit a pattern of recent terror attacks and increased the focus on already-heavy security for the Sochi Olympics in six weeks, U.S. scholars of Russia say.
No one immediately claimed responsibility for the bombing in Volgograd, but it came several months after Chechen rebel leader Doku Umarov called for attacks against civilian targets in Russia, including the Sochi Games. Umarov, the self-proclaimed emir of a terrorist group that calls itself the Caucasus Emirate, has called on Muslims to prevent the Olympics from occurring.
"An open question is how much authority he really has over these different groups," said Jeffrey Mankoff, deputy director and fellow at the Center for Strategic and International Studies' Russia and Eurasia Program. "A lot of the attacks seem to be inspired by Umarov but may not be directly controlled by him."
"If you are a terrorist group in the Caucasus, the Sochi Olympics are going to be a very inviting target," said Steven Pifer, director of the Brookings Institution's Arms Control and Non-Proliferation Initiative. "I think you're going to see at the Sochi Olympics a very heavy security presence."
The government has deployed tens of thousands of soldiers, police and other security personnel to protect the Games.
Some Muslim terrorists view the Olympics as a provocation, Mankoff said. Sochi, like other cities in the region, was conquered militarily in the middle of the 19th century. "They view it as a provocation on territory they consider stolen from Muslims in the 19th century," he said.
"It's a very tense environment," Mankoff said, noting that Muslim insurgency in the region began around 1990 in Chechnya. "Chechnya has been relatively pacified, but the insurgency has spread out to surrounding areas," he said.
Suicide bombings have rocked Russia for years, but many have been contained to the North Caucasus, the center of an insurgency seeking an Islamist state in the region. Until recently, Volgograd was not a typical target, but the city formerly known as Stalingrad has been struck twice in two months — suggesting militants may be using the transportation hub as a renewed way of showing their reach outside their restive region.
Volgograd, which lies close to volatile Caucasus provinces, is 550 miles south of Moscow and about 400 miles northeast of Sochi, a Black Sea resort flanked by the North Caucasus Mountains.
Through the day, officials issued conflicting statements on casualties. They said the suspected bomber was a woman, then reversed themselves and said the attacker could have been a man.
The Interfax news agency quoted unidentified law enforcement agents as saying footage taken by surveillance cameras indicated the bomber was a man. It reported that a torn male finger ringed by a safety pin removed from a hand grenade was found on the site of the explosion.
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. The officer was killed by the blast, and several other police were wounded.
"When the suicide bomber saw a policeman near a metal detector, she became nervous and set off her explosive device," Vladimir Markin, the spokesman for the nation's top investigative agency, said in a statement earlier in the day. He said the bomb contained about 22 pounds of TNT and was rigged with shrapnel.
Markin later told Interfax that the attacker could have been a man but said the investigation was ongoing. He said another hand grenade, which didn't explode, was found on the explosion site.
Markin said security controls prevented a far greater number of casualties at the station, which was packed with people as several trains were delayed.
Markin said 13 people and the bomber were killed on the spot, and the regional government said two other people later died at a hospital. About 40 were hospitalized, many in grave condition.
Earlier in the day, Lifenews.ru, a Russian news portal, posted what it claimed was an image of the severed head of the female attacker. It said the attacker appeared to have been a woman whose two successive rebel husbands had been killed by Russian security forces in the Caucasus.
Female suicide bombers, many of whom were widows or sisters of rebels, have mounted numerous attacks in Russia. They have been referred to as "black widows."
In October, a female suicide bomber blew herself up on a city bus in Volgograd, killing six people and injuring about 30. That attacker came from the province of Dagestan, which has become the center of the Islamist insurgency that has spread across the region after two separatist wars in Chechnya.
As in Sunday's blast, her bomb was rigged with shrapnel that caused severe injuries.
Chechnya has become more stable under the grip of its Moscow-backed strongman, who incorporated many of the former rebels into his security force. In Dagestan, the province between Chechnya and the Caspian Sea, Islamic insurgents mount near daily attacks on police and other officials.
The Kremlin replaced Dagestan's provincial chief this year, and the new leader abandoned his predecessor's attempts at reconciliation and efforts to persuade some of the rebels to surrender in exchange for amnesty.
Security camera images broadcast by Rossiya 24 television showed Sunday's moment of explosion, a bright orange flash inside the station behind the massive main gate followed by plumes of smoke.
A witness, Roman Lobachev, told Rossiya television that he was putting his bags on a belt for screening when he heard the sound of an explosion. "I heard a bang and felt as if something hit me on the head," said Lobachev, who survived the attack with minor injuries.
The bombing followed an explosion Friday in the city of Pyatigorsk in southern Russia, where a car rigged with explosives blew up on a street, killing three.
After Sunday's explosion, the Interior Ministry ordered police to beef up patrols at railway stations and other transport facilities across Russia.
In past years, Russia 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 bombings in 2010 and 2011, ordered a halt to attacks on civilian targets during the mass street protests against President Vladimir 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."
A group calling itself Anonymous Caucasus said in a statement Friday on the Caucasus rebel website kavkazcenter.com that it would launch cyber-attacks to avenge Russia's refusal to acknowledge the 19th-century expulsion of Chirkassians, one of the ethnic groups in the Caucasus.
The International Olympics Committee expressed its condolences over the bombing but said it was confident of Russia's security preparation for the games.
"At the Olympics, security is the responsibility of the local authorities, and we have no doubt that the Russian authorities will be up to the task," it said in a statement.
Russian authorities have introduced some of the most extensive identity checks and sweeping security measures ever seen at an international sports event.
Anyone wanting to attend the Games, which open 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 and check their identities upon arrival.
The security zone created around Sochi stretches approximately 60 miles along the Black Sea coast and up to 25 miles inland. Russian forces include special troops to patrol the forested mountains towering over the resort, drones to keep constant watch over Olympic facilities and speedboats to patrol the coast.
The security plan includes a ban on cars from outside the zone starting a month before the Games begin until a month after they end.