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Rescue workers are searching for survivors of a Russian express train crash who may be trapped in wreckage.

The government says 25 people are known to have died in the derailment, amid speculation that it may have been caused by a bomb blast.

Earlier, one official reported that 39 people had died, but that has now been denied by the authorities.

The train, travelling from Moscow to St Petersburg, crashed on Friday night near Bologoye town in Tver region.

About 90 people were injured as at least three carriages of the express train came off the tracks as it travelled on one of Russia's busiest rail links.

An investigation is under way into the cause of the crash. A small crater at the scene of the wreck has raised suspicions of sabotage.

"Several versions are being considered. It must be just a pit someone dug out. Or the crater was left by an explosive device," a law enforcement official was quoted by Itar-Tass news agency as saying.

Some witnesses heard a loud bang before the crash, Russian media reports said.

In 2007, a bomb on the same line derailed a train, injuring nearly 30 passengers.