Russia's main investigative agency says a Volgograd bus explosion was caused by a suicide bomber and officials raised the death toll to 14.
The bombing comes a day after 17 people died in a railway station bombing also blamed on a suicide bomber.
Investigative Committee spokesman Vladimir Markin said in a statement that Monday's explosion on an electric bus involved a similar bomb, proving a link between the two attacks.
Public health officials said that at least 14 people were killed in explosion during morning rush hour.
Volgograd is about 700 kilometres away from Sochi and is a key transportation hub to the Games.
No one has claimed responsibility for either bombing, but they came a few months after Chechen rebel leader Doku Umarov threatened new attacks against civilian targets in Russia, including Olympics in Sochi in February.