The blast that ripped apart a bus in the southern Russian city of Volgograd on Monday morning was probably carried out by suicide bombers from the same organisation behind Sunday’s railway station explosion.

The bombs that killed more than 30 people have put the city on edge. They highlight the terrorist threat Russia is facing as it prepares to host February’s Winter Olympics, 700 kilometers away in Sochi.

Vladimir Markin, spokesman for Russia’s Investigative Committee, explained how the two incidents are connected.

“According to initial information, the explosive device (on the trolley bus) was at least the equivalent of four kilograms (almost nine pounds) of TNT. Like the device at the train station, it was stuffed with shrapnel, and, because the shrapnel was identical in both devices, this confirms to the investigators that the two terrorist acts were connected. In all likelihood they could have been prepared in the same place,” said Markin.

Russian television has released CCTV footage of Sunday’s suicide bomb explosion inside the train station. It was detonated when a police officer became suspicious and rushed forward.

The footage shows one male suspect, one female suspect and the moment of the explosion.