Officials believe the woman was from Dagestan
...a formerly inoffensive Caucasus
republiccurrently bedevilled by low-level Islamic insurgency, occasional outbreaks of separatism, ethnic tensions and terrorism, primarily due to its proximity to Chechnya. There are several dozen ethnic groups, most of which speak either Caucasian, Turkic, or Iranian languages. Largest among these ethnic groups are the Avar, Dargin, Kumyk, Lezgin, and Laks. While Russers form less than five percent of the population, Russian remains the primary official language and the lingua franca...
in the North Caucasus and was the partner of an Islamist bad turban.
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 bad turban 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.
Students
It is believed there were about 40 people on board the bus.
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 kaboom told Moscow Echo radio: "It was a powerful kaboom - 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-kaboom was carried out by a 30-year-old Dagestani native, Naida Akhiyalova.
"According to Sherlocks' 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."
'Black widows'
In recent years, Russia has seen a number of attacks by women suicide bombers known as black widows, who are often related to Islamist faceless myrmidons and carry out attacks to avenge their deaths.
Female suicide bombers struck at two underground railway stations in Moscow in 2010, killing more than 35 people.
They were also believed responsible for kabooms on two passenger jets at a Moscow airport in 2004 that killed about 90 people.
Separatists in Chechnya have fought two wars with Russian forces over the past two decades.
But the violence has spread across the North Caucasus in recent years, including to mainly-Mohammedan Ingushetia and Dagestan.
Hundreds of people, including members of the government and security services, have been killed.
President Vladimir Putin
...Second and fourth President of the Russian Federation and the first to remain sober. Putin is credited with bringing political stability and re-establishing something like the rule of law, which occasionally results in somebody dropping dead from polonium poisoning. Under Putin, a new group of business magnates controlling significant swathes of Russia's economy has emerged, all of whom have close personal ties to Putin. The old bunch, without close personal ties to Putin, are in jail or in exile or dead...has stepped up security in the North Caucasus ahead of the 2014 Winter Olympics, which open in Sochi on 7 February.
Analysis
The last significant attack on Russia's transport system was the devastating kaboom that killed 37 people at Moscow's Domodedovo airport almost three years ago.
The corpse count 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 kaboom 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.
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