Authorities in southern Russia say a female suicide bomber blew herself up at a train station in the southern city of Volgograd, killing at least 16 people and wounding dozens.
An investigative committee spokesman says the woman set off the bomb in front of the metal detectors at the station entrance early Sunday morning.
Television footage shows a big orange fireball just inside the station as smoke pours out of the windows.
No one has claimed responsibility for the attack. But authorities found what they say was the bomber's severed head. They identify her as coming from Dagestan, a republic in the nearby volatile North Caucasus.
U.S. Ambassador to Russia Michael McFaul extendedhis condolances to the dead and injured in what he called a terrorist attack.
This attack comes weeks before the Winter Olympics open in Sochi, about 650 kilometers south of Volgograd. Islamist militants have threatened to disrupt the Games.
An attack in Volgograd by a female suicide bomber on October 21 killed five people and wounded 30. Investigators also identified her as coming from Dagestan.
Dagestan is the epicenter of an ongoing Islamist insurgency in the North Caucasus.
In early July, the leader of the Islamist insurgency in the North Caucasus, Doku Umarov, declared an end to a moratorium on attacks on Russian civilian targets that he had announced the previous year.