A number of details remain unclear, but at least 16 people are dead and scores more wounded after a suicide bombing at a railway station in southern Russia.
The casualties include the bomber, who officials initially said was a woman. But the Interfax news agency later quoted law enforcement agents as saying that surveillance footage indicated that the bomber was a man.
The attack took place in Volgograd, which lies close to the volatile Caucasus provinces.
No one immediately claimed responsibility for the bombing, but it comes several months after a Chechen rebel leader (Doku Umarov) called for new attacks against civilian targets in Russia, including the Winter Olympics set for February in Sochi (SOH'-chee).
Russia has been hit by suicide bombings for years, but most have been in 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 transportation hub has now been struck twice in two months.
At least 13 people were killed in a terror attack by a suspected female suicide bomber at a railway station in Russia Sunday, officials said - the second deadly attack in three days as the country prepares to host the Winter Olympics.
The National Anti-Terrorist Committee said the explosion, at the central railway station in the city of Volgograd, was an act of terror and that a criminal investigation has been launched.
The attack, which happened at about 1 p.m. local time (4 a.m. ET), heightens concern about terrorism ahead of February's Olympics in the Black Sea resort of Sochi.
On Friday, a car bomb killed three people in the southern Russian city of Pyatigorsk, 170 miles east of Sochi. In October, another female suicide bomber was blamed for a bus explosion in Volgograd that killed five people.
The Committee said preliminary findings after Sunday's blast showed that the suspect was female, but federal security services were still investigation.
"Today at around 1300 an unconfirmed explosive device was detonated in front of the metal detector at the entrance to the train station in Volgograd,” the Committee said on its website. “The preliminary findings show that it was activated by a female suicide bomber."
Russian Investigative Committee Representative Vladimir Markin told state news agency Interfax that 13 were dead.
Other reports in Russian media said that as many as 18 were killed, and at least 50 injured.