A bus exploded in the Russian city of Volgograd on Monday, killing more than six passengers - officials wager a female suicide bomber was responsible.
The blast occurred shortly after 10:00 GMT (5:00 a.m. EST), and left more than 30 people injured.
According to the BBC, law enforcement officials maintained the woman was from Dagestan in the North Caucasus, and was working with a radical Islamist. A rise in membership of an Islamist insurgency in the area has created serious unrest in the past few years. BBC's Steve Rosenberg reported from Moscow that some fear the extremist groups could be working to ramp up attacks in Russia.
Around 40 people were on the bus when a self-explosive detonated on board. All buses in the Volgograd area were sent back to their depots, and searched for any bombs.
One man who interviewed with presenters on Moscow Echo radio recounted his experience with the bomb, which he and his daughter both managed to survive.
"It was a powerful explosion," he said, "a huge blast. There were lots of students on the bus."
Another man who was driving behind the bus reported to Rossyia-24 television that the "horrible sight" could be seen from many cars down.
"There was a blast - a bang - all the glass flew out of the windows," he said. "The cloud of smoke quickly dissipated and then I saw people start to fall out and run to escape the bus."
Local reports suggested the explosion occurred after a gas canister exploded on the bus.
"A criminal case has been opened under articles outlining terrorism, murder and the illegal use of firearms," Investigative Committee member Vladimir Markin told RIA Novosti news agency.
Interfax news agency later quoted Markin as saying that the suspected bomber was a woman from Dagestan.
"According to preliminary information, the self-explosion was carried out by a 30-year-old Dagestani native, Naida Akhiyalova," he said. "According to investigator's 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."
Over the past few years, female suicide bombers commonly referred to as "black widows" have cropped up around Russia. They usually are in cahoots with radical extremists who carry out attacks to avenge the deaths of loved ones, the BBC reported.
