Russia's main investigative agency has announced the conviction and sentencing of four men accused of involvement in twin suicide bombings in Volgograd a year ago.

The bombings of a train station and an electric trolleybus in the southern city killed 34 people and heightened security fears ahead of the 2014 Sochi Winter Olympics.

The Investigative Committee said Friday that a Volgograd court has found two men guilty of complicity in terrorism for sheltering the suicide bombers as they prepared the explosives, and sentenced them to 19 years in prison.

The federal agency said two other men who transported the suicide bombers to Volgograd were sentenced to five years and 10 months.

All four are from Dagestan, a republic in Russia's North Caucasus that is the center of an Islamic insurgency.