At least 17 people were killed and 34 were wounded Sunday by a female suicide bomber at a railway station in southern Russia, officials said, heightening concern about attacks ahead of February's Olympics in the Black Sea resort of Sochi.
No one immediately claimed responsibility for Sunday's attack in Volgograd. It came several months after Chechen rebel leader Doku Umarov called for new attacks against civilian targets in Russia, including the Sochi Games.
Volgograd is 550 miles south of Moscow and about 600 miles northeast of Sochi, a Black Sea resort flanked by the North Caucasus Mountains.
The distance of the target from Sochi reflects how strict security has become around Moscow and the Olympics site itself, Barnard College political science professor Kimberly Marten told Al Jazeera.
"No question it's related to the Olympics," Marten said.
Suicide bombings and other attacks linked to Islamic rebels roaming the North Caucasus have rocked Russia for years. The government has deployed tens of thousands of soldiers, police and other security personnel to protect the Olympics, President Vladimir Putin's pet project, and organizers have pledged to make the Sochi Games the "safest Olympics in history."
Still, sports economist and Smith College professor Andrew Zimbalist told Al Jazeera on Friday that the fear of attacks would deter some tourists from attending the Games, which are expected to cost upward of $60 billion — the most in Olympic history, and well over 30 percent more than the 2008 Beijing Olympics.
Vladimir Markin, the spokesman for the nation's top investigative agency, the Investigative Committee, said the suicide bomber in Sunday's attack detonated her explosives in front of a metal detector.
Russia's Health Ministry said about 50 people were injured.
In October, a female suicide bomber blew herself up on a city bus in Volgograd, killing six people and injuring about 30. Officials said the attacker came from the province of Dagestan, which has become the center of an Islamist insurgency that has spread across the region after two separatist wars in Chechnya.
After the October incident, Russian authorities said they had started taking saliva samples from religiously conservative women in the area, in order to identify the women if they became suicide bombers.
Russian state television reported that Sunday's explosion occurred at the security gate at the station's entrance. The Interior Ministry said one police officer died in the explosion and three others were wounded.
Images caught by a security camera facing the station, broadcast by Rossiya 24 television, showed the moment of explosion: a bright orange flash inside the station behind the main gate followed by plumes of smoke.
"We heard a loud bang from behind, saw a bright flash and fell on the floor," area resident Svetlana Demchenko, who witnessed the explosion, was quoted as saying by the ITAR-Tass news agency.
Numerous ambulances were parked outside the station, and several motionless bodies could be seen on the pavement.
On Friday, three people were killed when a car rigged with explosives blew up on a street in Pyatigorsk, the center of a federal administrative district intended to stabilize the North Caucasus region.
Russia in past years has seen a series of 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 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."
Al Jazeera and The Associated Press