MAKHACHKALA, Russia — Russian investigators say security forces killed a militant who may have helped train the two suicide bombers who struck the southern city of Volgograd.

The bombings of a train station and an electric trolleybus in late December killed 34 people and heightened security fears ahead of the Sochi Winter Olympics, which begin on Friday.

Russia's counterterrorism agency last week named the suspected bombers and said they were members of a terrorist group based in Dagestan, a republic in Russia's North Caucasus region that is the center of an Islamic insurgency. Two suspected accomplices have been arrested.

On Wednesday, a spokesman for the Russian investigative agency said the militant killed in a shootout in Dagestan may have been involved in training the bombers and sending them to Volgograd.