Two back-to-back suicide bombings in Volgograd have now claimed at least 30 lives and are believed to be acts of terrorism, and although no groups have come forward to claim responsibility for the attacks, officials say the bombs were the same.

The first explosion rocked a large railroad station–at what investigators believe was outside security checkpoints–on Sunday, killing at least 15 people. The blast was caused by a shrapnel bomb which injured more than three dozen, and officials say they expect the death toll to rise as many of the victims are in critical condition. Early Monday morning during rush hour, a second bomber hit a trolley bus, killing 14 people.

Investigators have not named a suspect, but according to the New York Times, Chechen rebel Doku Umarov swore earlier this year to bring violence to Sochi, where the Olympic games will be held in February.

“They plan to hold the Olympics on the bones of our ancestors, on the bones of many, many dead Muslims, buried on the territory of our land on the Black Sea,” he said.

According to Russian Olympic Committee chief Alexander Zhukov, every security measure that can be put into place at Sochi has been made, including drones which will patrol the Olympic facilities, boats to patrol the coast, and troops to keep watch in the mountains around the area. Anyone who purchases a ticket to the games will have to give their passport details and will be screened and checked in at arrival.

“There’s every belief they’ll make it secure and do whatever it takes to do that,” said an American senior law enforcement official. “But it is a large country, and these groups can get a lot of bang for their buck if they are able to do something in the country, wherever it is, during the Olympics.”

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