A bomb blast tore through a bus in the southern Russian city of Volgograd on Monday, authorities said, killing at least five people in the deadliest such attack outside Russia's volatile North Caucasus region in nearly three years.
The explosion also wounded 17 people, seven of whom were in grave condition, a spokesman for the local investigators said.
After a series of conflicting reports about the cause, the National Anti-Terrorism Committee said it was a bomb.
"Today at 2:05 p.m., an unknown explosive device detonated on a passenger bus in the city of Volgograd, causing human casualties," the committee, Russia's top counter-insurgency agency, said in a statement.
It did not assign blame and there was no immediate claim of responsibility.
Insurgents who say they are fighting to create an Islamic state in Russia's mostly Muslim North Caucasus have carried out deadly bombings inside and outside the region, made up of several provinces along Russia's southern border.
The insurgents claimed responsibility for a suicide bombing that killed 37 people at Moscow airport in January, 2011, and two nearly simultaneous suicide bombings that killed 40 people on the Moscow subway in 2010.
Volgograd is a city of around one million people that lies 900 km (560 miles) southeast of Moscow and a few hundred kilometres north of the North Caucasus and Black Sea resort city of Sochi, where Russia will host the 2014 Winter Olympics.
President Vladimir Putin has staked his reputation on the Games and ordered authorities to boost security in the North Caucasus, where the Islamist insurgency is rooted in two post-Soviet wars pitting Chechen separatists against the Kremlin.