A bus explosion in Volgograd was caused by a suicide bomber, Russia's main investigative agency said, as officials raised the death toll to 14.
The bombing comes a day after 17 people died in a railway station bombing in the city 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 the explosion during morning rush hour.
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 the Winter Olympics in Sochi in February.