Authorities believe the blast on Thursday evening in Voronezh is not terror-related.
Two people were killed and 17 more were injured on Thursday after an explosion on a bus in southwestern Russia.
The region's governor, Alexander Gusev, said that the first victim, a woman, died shortly after the explosion. A second woman died from her injuries in hospital.
Seventeen people were also wounded and several are in serious conditions, Gusev also said.
The bus driver told state broadcaster Rossiya-24 that 35 people were on board at the time of the tragedy. The blast took place when the vehicle was at a bus stop near a shopping centre.
"The explosion was very loud, then everyone started running to escape," one witness, Larissa, told the state news agency RIA Novosti, adding that "the whole road was strewn with broken glass".
Authorities believe the blast on Thursday evening in Voronezh, a city of 1 million about 450 kilometres south of Moscow, is not terror-related as no trace of chemical explosives was found.
They say it was caused by leaky gas equipment.
An investigation into "improper provision of services" resulting in "serious harm to people's health" has been opened, the Russian Investigative Committee said in a statement.
But the company that operated the bus said the vehicle had a diesel engine and no gas equipment and insisted it was in good technical condition.
The National Anti-Terrorist Committee said all options were being considered and experts continued to analyse fragments from the bus.