A suspected female suicide bomber set off a blast on a bus packed with students in southern Russia on Monday, killing at least five people and raising security fears less than four months before the Winter Olympic Games.
Officials said the explosion that injured nearly 30 people went off at around 2:00 pm (1000 GMT) in the Volga River city of Volgograd, about 900 kilometres (560 miles) southeast of Moscow.
Footage broadcast on state television showed a green and white city bus standing mangled in the middle of the street, its windows blown out on the left side.
"It was an unidentified explosive device," a spokesman for the National Anti-Terror Committee said by telephone.
The Investigative Committee, Russia's equivalent to the US Federal Bureau of Investigations, said officials had opened a formal terror probe.
"A criminal case has been opened under articles outlining terrorism, murder and the illegal use of firearms," the Investigative Committee said in a statement.
An official at the Investigative Committee said police had found the identity papers of a suspected female suicide bomber who they believe was the wife of a rebel commander fighting in Russia's restless North Caucasus.
"She had recently adopted Islam," the unnamed Investigative Committee source told Russian news agencies.
Officials said the packed bus was carrying about 40 passengers at the time of the explosion.
"There were lots of students on the bus," a man named Vladimir, whose daughter survived the explosion, told Moscow Echo radio from Volgograd.
"It was a powerful explosion -- a huge blast," he said.
"The bus was torn to pieces. When I came to pick up (my daughter), half the bus was simply not there."
A regional emergencies ministry spokeswoman told AFP that 17 people were hurt in the explosion, although some Russian news agency reports put the number of injured as high as 27.
Initial reports had also put the death toll at six, although most local and national security agencies had later revised the number of dead down to five.
"All those who need help are getting full assistance," an unnamed Volgograd government official separately told the Interfax news agency.
"We have gathered the required amount of medicine, and called up additional doctors," the official said.
Security remains a concern throughout southern Russia ahead of the February 7-23 Winter Olympic Games in the Black Sea resort of Sochi, which is located next to the volatile North Caucasus.
Russia fought two post-Soviet wars in the North Caucasus republic of Chechnya, and has since seen violence spill over to neighbouring Muslim regions such as Ingushetia and Dagestan.
Female suicide bombers are often referred to in Russia as "black widows" -- women who seek to avenge the deaths of their family members in North Caucasus fighting by targeting Russian civilians.
Two female suicide bombers set off blasts at two Moscow metro stations in March 2010 that killed more than 35 people.
So-called black widows were also responsible for taking down two passenger jets that took off from a Moscow airport within minutes of each other in 2004, killing about 90 people.
Several also participated in the September 2004 Beslan school hostage crisis that ended with the death of at least 335 people, about half of them children.