The authorities in Russia are treating the crash of an express train in which at least 25 people were killed as a possible act of terrorism.
A crater has been found near the track, raising suspicions that a bomb may have been used to derail the train.
The prosecutor-general has opened a criminal case on terrorism charges, Russian news agencies report.
The train crashed in remote countryside as the train travelled between Moscow and St Petersburg.
Hundreds of rescuers and officials have been working throughout the night at the scene near the town of Bologoye in Tver region.
Some reports say that as many as 39 people have died.
The train was carrying more than 650 people. More than 90 are in hospital, some of them taken there by helicopter.
The chief of state-owned railroad firm Russian Railways, Vladimir Yakunin, said investigators believed Friday evening's crash was caused by an act of terrorism.
"To put it simply, a terrorist attack" was the main line of inquiry being pursued by experts investigating the derailment, Mr Yakunin said on state television from the scene.
Some passengers reported a loud bang occurring just before the derailment.
And Russian television channels have been broadcasting a recording of a mobile phone call from the train driver to the emergencies ministry. The driver says there has been an explosion under the train.
The train, known as the Nevsky Express, was travelling on one of the busiest rail routes in Russia, and Friday evening is peak travel time.
In 2007, a bomb on the same line derailed a train, injuring nearly 30 passengers.
Two men suspected of having links to Chechen rebels were accused of planting a bomb next to the track.