Two suicide bombings in as many days have killed 31 people and raised concerns that Islamic militants have begun a terrorist campaign in Russia that could stretch into the Sochi Olympics in February. Russian and international Olympic officials insisted the site of the games, protected by layers of security, is completely safe.

The attacks in Volgograd, about 400 miles (650 kilometers) from Sochi, reflected the Kremlin’s inability to uproot Islamist insurgents in the Caucasus who have vowed to derail the games, the pet project of President Vladimir Putin.

No one has claimed responsibility for Sunday’s blast at the Volgograd railway station or Monday’s bus explosion in the city, but they came only months after Chechen rebel leader Doku Umarov threatened new attacks on civilian targets in Russia, including the Olympics.

In addition to the dead, the bombings wounded 104 people, according to Russia’s Health Ministry. As of late Monday, 58 remained hospitalized, many in grave condition.

Suicide bombings have rocked Russia for years, but the insurgents seeking to create an Islamic state have largely confined their attacks to the North Caucasus region in recent years. The blasts in Volgograd signaled that militants want to show their reach outside their native region.

Matthew Clements, an analyst at Jane's, said Caucasus militants could be targeting major transportation hubs like Volgograd to embarrass the Kremlin and discourage attendance at the Feb. 7-23 Olympics. (Associated Press)