An elite passenger train carrying hundreds of people from Moscow to Saint Petersburg was derailed by a bomb attack that left dozens dead and nearly 100 injured, officials said on Saturday.
The attack occurred late Friday about 400 kilometres (250 miles) northwest of Moscow in a forest where the same train, the Nevsky Express, popular with tourists and commuters, was hit by a similar bomb attack in August 2007.
"We are indeed talking about a terrorist attack," Vladimir Markin, spokesman for the federal investigative committee, was quoted as saying by the Interfax news agency.
The committee said in a statement that it had recovered bomb fragments at the scene of the derailment, and a top official told President Dmitry Medvedev the disaster had been caused by a bomb blast.
"It appears that an improvised explosive device with the force of around seven kilogrammes of TNT was detonated," the head of the FSB security service said in a meeting with Medvedev shown on state television.
Markin gave a similar explanation, saying evidence found at the scene suggested "an explosive device was detonated along the train's path."
Mangled, overturned carriages were strewn across the tracks and down the railway embankment as scores of orange-vested rescuers searched throughout the day for more victims in the wreckage and worked to repair the railway line.
Nevsky Express tickets are too expensive for most Russians and the train, the fastest way to travel from central Moscow to central Saint Petersburg, is used mostly by middle-class or wealthy Russians and foreign tourists.
Witnesses including passengers on the train and locals living near the site said they heard a loud boom just before the train went off the rails, and police told AFP at the site there was a large crater under the track.
"It was very loud, and we were very frightened," recounted one elderly woman whose house lies near the heavily-traveled railway line.
The head of state firm Russian Railways said later that a second bomb had partially exploded Saturday in the vicinity of the disaster, but the device malfunctioned and no one was hurt, news agencies said.
There was no immediate credible claim of responsibility for the attack.
Medvedev ordered several law enforcement agencies to investigate the train attack thoroughly and to assist people affected by it.
"Make sure there is no chaos, because the situation is already tense," Medvedev said in a video conference shown on state television.
Russian Interior Minister Rashid Nurgaliyev said police had evidence suggesting that several persons took part in the attack.
"There are many elements that can point us in the right direction to solve this crime," Nurgaliyev was quoted by RIA Novosti news agency as saying.
In a meeting later with a government commission specially formed to manage the disaster, also shown on state television, Medvedev expressed condolences for families of the victims and stood for a moment of silence.
Officials said 26 people had been killed and 96 others injured.
One official, Alexander Basulin of the emergency situations ministry, was quoted by ITAR-TASS news agency as saying that 39 people died, but his toll was not corroberated by other officials.
The emergency situations ministry said 18 passengers were still unaccounted for, but Minister Sergei Shoigu said this did not mean they were dead, only that they could not immediately be located.
Andrei Abramenko, a police officer who happened to be travelling on the train, described a picture of human suffering.
"Two wagons were completely overturned.... Several people were completely crushed under the metal. I heard screams, moaning," Abramenko said on the Vesti-24 television news channel.
At least three foreigners were on the train, ITAR-TASS reported, citing a source at Russian Railways. Other reports said one Italian and two Finns were among the 660 passengers.
In Washington, the White House said it was "deeply saddened by the terrible loss of life and injuries" from the incident.
The European Union said it "deplores the tragic loss of lives" in the disaster and "supports Russia in its efforts to bring clarity into the causes of this tragic event."
Russia's main television stations have announced they plan to halt broadcasts of entertainment programmes for several days as a mark of respect to the victims of the attack, Interfax news agency reported.