A bomb blast caused the derailment of a Russian express train, killing at least 26 people, intelligence officials say.
The Nevsky Express derailed in remote countryside on Friday night as the train travelled between the capital Moscow and St Petersburg.
Investigators found "elements of an explosive device" at the scene, the Russia's federal investigative committee said in a statement.
A senior intelligence official said a bomb had derailed the locomotive.
Alexander Bortnikov, head of Russia's domestic intelligence service told Russian President Dmitry Medvedev that a bomb equivalent to seven kilos (15 lb) of TNT had been detonated, Reuters reported.
There was no immediate confirmed claim of responsibility for the attack.
Russia's prosecutor-general has opened a criminal case on terrorism charges, Russian news agencies report.
Hundreds of rescuers and officials worked 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.
Some passengers reported a loud bang occurring just before the derailment.
And Russian television channels broadcast a recording of a mobile phone call from the train driver to the emergencies ministry.
"There was an explosion under the locomotive," he said. "I do not know what we hit. We are derailed. The locomotive and carriages, I do not know yet what else, everything is in smoke. "
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.
BBC News - Russia train crash 'caused by bomb'