At least 39 people were killed and nearly 100 injured when a Russian express train was derailed late on Friday in what the head of the national railway company said could have been a bomb attack.
Russian prosecutors said on Saturday they had opened a criminal case on charges of terrorism and illegal possession of explosives but did not say who they suspected of responsibility or what their motives might have been.
"... a blast from an explosive device is one of the main explanations for the Nevsky Express incident," Russian Railways chief Vladimir Yakunin told reporters at the scene.
He later told Vesti-24 state television that investigators believed the blast was "to put it simply, an act of terrorism."
The Nevsky Express, carrying 661 passengers from Moscow to St Petersburg, was derailed at 9.34pm (1834 GMT) near the village of Uglovka about 350km (200 miles) north of Moscow.
A Reuters photographer saw soldiers carrying four body bags away from the scene where rescue workers cut through the tangled steel to search for survivors in two wrecked train carriages.
Emergencies minister Sergei Shoigu was told by a ministry official in a video conference shown live on Vesti-24 that more bodies had been pulled from wrecked carriages and the death toll had risen to 39.
Shoigu later said that 25 people had been confirmed dead but the toll could rise as 18 people were still unaccounted for. Health Minister Tatyana Golikova said half the injured people were in serious condition.
The derailment is Russia's worst train disaster for years and could raise fears of a surge in attacks on the Russian heartland by rebels from the North Caucasus.
"The so-called Chechen trace is traditionally viewed as the main one during investigations of such disasters," said Alexei Mukhin of the Centre for Political Information — but added that outdated infrastructure also caused major accidents in Russia.
Yakunin said Friday's blast looked similar to an explosion in August 2007 that derailed a Nevsky Express train on the same line and injured at least 30 people. Prosecutors at that time arrested two residents of Ingushetia and charged them with helping to carry out the attack.
Prosecutors said they believed ex-soldier Pavel Kosolapov, a former associate of the late Chechen rebel leader Shamil Basayev, was the mastermind behind the 2007 blast. Kosolapov is still on the run.
Interfax news agency said a 1-metre (3-ft) wide crater had been found next to the railway track on Friday, though Reuters reporters at the scene did not see one.
A railway official who asked not to be named said a witness had reported hearing a loud bang, but another passenger told reporters in St Petersburg there had been no blast.
The derailment has delayed 27,000 people as transport officials try to divert trains onto smaller lines.
"You must make sure there is no chaos," president Dmitry Medvedev told ministers through a video conference without mentioning the cause of the crash.
In Washington, White House spokesman Robert Gibbs said, "We are deeply saddened by the terrible loss of life and injuries resulting from the reported derailment of a train between Moscow and St Petersburg."
