Richard Galpin reports on the causes of the train disaster
A bomb blast caused the Russian train crash in which at least 26 people were killed, intelligence officials say.
The Nevsky Express derailed with nearly 700 on board as it ran through remote countryside between the capital Moscow and the second city, St Petersburg.
Investigators found "elements of an explosive device" at the scene of Friday's attack, a statement said.
Officials said a second, less powerful device went off on Saturday near the site of the first, but no-one was hurt.
There was no immediate confirmed claim of responsibility for the blast on Friday evening, which hit a train popular with government officials and business executives at peak travel time.
"Criminology experts say, on the basis of preliminary information, that an improvised explosive device, equivalent to 7kg (15 lb) of TNT, had gone off," said Alexander Bortnikov, head of Russia's domestic intelligence service.
At least three of the 14 carriages left the tracks as the train reportedly approached speeds up 200 km/h (130mph).
Russia's prosecutor-general has opened a criminal case on terrorism charges, Russian news agencies say.
If terrorism is confirmed as the cause, observers say the derailment would represent the deadliest attack outside the volatile North Caucasus region for five years.
'Attack on elite'
I think we can expect the Russian authorities to come up with some name soon, because this attack is politically very embarrassing
Pavel Felgenhauer Novaya Gazeta newspaper
Pavel Felgenhauer, defence correspondent for Russia's Novaya Gazeta newspaper, told the BBC News website the key suspects for investigators would be "either militants from Russia's North Caucasus region or nationalist extremists, pro-Nazi groups".
"I think we can expect the Russian authorities to come up with some names soon, because this attack is politically very embarrassing.
"This is an expensive, high speed train, used by an elite which has been pushing to transfer parts of government functions to St Petersburg. We already have reports of several high-ranking government and local officials among the dead.
"Whoever is responsible, this attack clearly seems aimed not so much at the public, but directly at the ruling class."
Hundreds of rescuers and officials worked through the night at the scene near the town of Bologoye, about 400km (250 miles) north-west of Moscow.
Some reports say as many as 39 people have died.
The train was reported to be carrying around 650 passengers and two dozen or so staff.
About 90 people are in hospital, some taken there by helicopter.
Many of the injured are said to be in a serious condition.
According to some reports, the scene of the crash, in wooded countryside, was so remote it took emergency services two hours to get there.
Passengers spoke of a loud bang just before the derailment.
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. "
Federal prosecutors' spokesman Vladimir Markin told Itar-Tass news agency that a crater found at the site was 1.5m (5ft) wide and 0.7m deep.
"Indeed this was a terrorist attack," he said.
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.
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