At least 16 people were killed on Sunday when explosives went off at the main railway station in the Russian city of Volgograd, 970 kilometers (602 miles) southeast of the capital, Moscow.
The attack is likely to fuel fears of attacks by Islamist militants as Russia prepares to host the 2014 Winter Olympics in the Black Sea city of Sochi in less than six weeks' time.
Regional officials said the suicide bomber detonated explosives near metal detectors placed at the entrance to the station, which was packed with people travelling for the New Year holiday.
Dozens of people were also said to have been wounded in the blast, which reportedly blew out the windows and doors of the three-storey building. Footage captured by a nearby camera the moment of the blast showed a huge orange fireball fill the hall of the station and clouds of grey smoke pour out of the shattered windows as people ran into the streets.
Sochi is located near North Caucasus regions such as Dagestan and Chechnya, where violence has occurred on a near-daily basis in a long-running insurgency by Islamist militants.
The Chechen warlord Doku Umarov in July issued a video in which he urged militants to use "maximum force" to prevent Russian President Vladimir Putin from staging the Olympics.
Questions over identity of attacker
Throughout the day there were conflicting reports about the identity of the attacker.
Officials initially said a female suicide bomber had blown herself up after a police officer approached her near the metal detectors because she looked suspicious.
A Russian website with ties to security agencies, lifenews.ru, published a picture of what is said what the head of the young female bomber. It identified her as a Dagestani woman whose two successive husbands had been killed by Russian security forces in the Caucasus.
However a spokesman for Russia's Investigative Committee, Vladimir Marki, later told the Interfax news agency that the attacker may have been a man. He said there had been no claim of responsibility and the investigation was still ongoing.
Female suicide bombers have mounted numerous attacks in Russia, including a deadly Moscow theatre siege in 2002. Often wives or sisters of rebels killed in fighting in the regions, they are commonly known as "black widows."
Volgograd saw another deadly attack only recently, when a female suicide bomber killed six people aboard a crowded bus on October 21.
Sunday's attack is the deadliest such explosion outside the nearby volatile North Caucasus region in nearly three years.
It also comes just two days after a car bomb killed three people in the southern Russian city of Pyatigorsk, 270 kilometers east of Sochi.
ccp, tj/pfd (AFP, Reuters, AP)