Authorities in Russia say a train crash that killed at least 39 people and injured nearly 100 was caused by a bomb.The intelligence service has concluded it was an explosion with the equivalent to seven kilos of TNT that pushed the express train off the tracks late on Friday evening. Prosecutors have already opened a criminal case on charges of terrorism and possession of explosives but have not said what the motive behind the attack could have been or who they suspect. The Nevsky Express was traveling from Moscow to St. Petersburg with more than 600 people on board. It came off the tracks near the town of Bologoye. A search has revealed a wide crater next to the track. Igor Pechnikov was on the train: “I was in the second carriage. One of the two that derailed. I heard a crack, the carriage started moving left, my chair came off and I was thrown to the middle of the carriage. The carriage was crushed.” Around 27,000 people have been delayed as transport officials try to divert trains along smaller lines. Two years ago, 30 people were injured when a train operating on the same line was derailed by a bomb said to have been placed by Chechen rebels.