A suspected female suicide bomber has set off explosives on a bus in the southern Russian city of Volgograd, killing at least six people.
Officials believe the woman was from Dagestan in the North Caucasus and was the partner of an Islamist militant.
The blast, which happened just after 14:00 (10:00 GMT), also injured more than 30 people, some of them seriously.
An Islamist insurgency in the North Caucasus region has led to many attacks there in recent years.
The BBC’s Steve Rosenberg in Moscow says the bombing has raised fears that militant groups may be planning to step up attacks in Russia in the run-up to the Winter Olympics in the Black Sea resort city of Sochi next February.
It is believed there were about 40 people on board the bus.
The last significant attack on Russia’s transport system was the devastating bomb blast that killed 37 people at Moscow’s Domodedovo airport almost three years ago.
The death toll from today’s attack was much lower. But with just four months to go to the Sochi Winter Olympics, its significance will not have gone unnoticed in the Russian government or the international Olympic movement.
If indeed the explosion is linked to the North Caucasus region of the Russian Federation and its troubled republics of Dagestan, Chechnya and Ingushetia, it is a reminder of how easy it is for a single suicide bomber to cause multiple deaths in any part of Russia.
The speedy identification of a suspect will not reassure anyone that the situation is under control, and everyone will be hoping this was a one-off and not the start of a bombing campaign.
Our correspondent says that all buses in Volgograd have been ordered back to their depots to be searched for any sign of explosives.
One man whose daughter survived the explosion told Moscow Echo radio: “It was a powerful explosion - a huge blast. There were lots of students on the bus.”
Another man, who was driving behind the bus, told Rossiya-24 television: “There was a blast - a bang - all the glass flew out of the windows. The cloud of smoke quickly dissipated and then I saw people start to fall out and run out to escape the bus. It was a horrible sight.”
Earlier reports had suggested the blast might have been caused by an exploding gas canister.
Volgograd lies about 900km (560 miles) south of Moscow and 650km north of the North Caucasus.
Vladimir Markin, of the Investigative Committee - Russia’s equivalent of the FBI - told the RIA Novosti news agency: “A criminal case has been opened under articles outlining terrorism, murder and the illegal use of firearms.”
Mr Markin was later quoted by the Interfax agency as identifying the suspected suicide bomber as a woman from Dagestan.
He said: “According to preliminary information, the self-explosion was carried out by a 30-year-old Dagestani native, Naida Akhiyalova.
“According to investigators’ information, the woman entered the bus at one of the bus stops and, almost right after that, the bomb went off. That is also confirmed by one of the passengers who survived.”
The suicide blast that devastated a bus in Volgograd was meant to take place in Moscow, a source in the Russian Investigative Committee told RT. The investigation also revealed that the bomber had conspirers in the capital who are on the wanted list.
According to the source, the investigation shows that Naida Asiyalova - the 30-year-old native of the Republic of Dagestan who is believed to be the woman behind the Monday bus bomb blast in Volgograd - conspired with her partner, Dmitry Sokolov, and two other militants to carry out a terrorist attack in the Russian capital.
Two of the conspirers, Ruslan Kazanbiyev and Kurban Omarov, both 25, had already arrived to Moscow and were waiting for Asiyalova, RT’s source said. Both men are wanted in Russia for carrying out a twin terrorist bombing in Dagestan in May 2012, in which 14 people were killed and over 100 injured.
Sokolov was also expected by the terrorists to arrive in Moscow, the source added. However, for some reason Asiyalova detonated her explosive device in a passenger bus in Volgograd, killing herself and six others, and injuring over 30.
Witnesses said the blast went off soon after Asiyalova entered the bus. They did not mention that she had any companions with her.
The police have reportedly put Kazanbiyev, Omarov, and Sokolov on the federal wanted list, and will distribute the terrorist suspects’ photos among all police stations and checkpoints in Russia.