The police force in Volgograd has been depleted in recent months as some 600 officers were redeployed to Sochi to tighten security around Olympic sites, a local police officer told Reuters.
More attacks can be expected before the Olympics and cities in southern Russia where the Games are not being held are easier targets than Sochi, said Alexei Filatov, deputy head of the veterans' association of the elite Alfa anti-terrorism unit.
"The threat is greatest now because it is when terrorists can make the biggest impression," he said. "The security measures were beefed up long ago around Sochi, so terrorists will strike instead in these nearby cities like Volgograd."
Monday's blast was so powerful it blew out the third-storey windows of a nearby apartment building, and witnesses said passengers were flung from the bus by the blast.
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"We ran outside. There was smoke and people were lying in the street. The driver was thrown far. She was alive and moaning ... Her hands and clothes were bloody," said Olga, a clerk in a shop near the scene of the blast.
A city of about 1 million, Volgogradis one of the venues for the 2018 soccer World Cup, another high-profile sports event Putin has helped Russia win the right to stage, and which will bring thousands of foreign fans to cities around Russia.
Sunday's attack was the deadliest to strike Russia's heartland since January 2011, when a male suicide bomber from the North Caucasus killed 37 people in the arrivals hall of a busy Moscow airport.
Investigators initially said a woman set off the bomb that filled the front of the railway station building with a massive orange fireball on Sunday, but later said the bomber could have been a man.