The Prosecutor-General's Office says it believes a bomb with the power of about two kilograms of TNT caused all 12 carriages of the Neva Express passenger train to come off the tracks.
Sergei Bednichenko, the chief prosecutor for Russia's Northwest Federal District, said today in televised remarks that the accident was caused by a homemade explosive device.
He said prosecutors have opened a criminal probe into terrorism.
"The cause of the incident is being investigated, and this is a task of the FSB," said Viktor Korsakov, a spokesman for the regional Moscow-St. Petersburg rail line. "I can only say that it was the result of unlawful interference in the functioning of the rail line. There was a bang and what happened afterward, nobody knows."
Two explosions reported
The train was heading from Moscow to St. Petersburg when it derailed near the village of Malaya Vishera in Novgorod Oblast, 500 kilometers northwest of the capital.
In an interview with RFE/RL's Russian Service, the driver of the train, Alexei Fedotov, said an explosion occurred under the train just after it passed Burga station, heading toward Malaya Vishera.
"Within 20 to 30 meters, there was a blast under my cab," Fedotov said. "Glass is shattering around me. Everything [is covered] in dust. Fire is all around. I have instantly realized that we have been blown up. I immediately apply the emergency break, reduce the speed, release the trolleys so that the wires don't get twisted. That's all. And then we fly about 700 meters [down the track]. I was driving at 185 kilometers per hour."
An RFE/RL correspondent in Novgorod Oblast reports that doctors arrived on the scene within 20 minutes of the accident, which occurred around 10 p.m. local time.
Some 30 people were hospitalized, and three remain in critical condition.
Russian President Vladimir Putin ordered Transportation Minister Igor Levitin to spare no effort to help the victims.
"You, right now, together with the Health Ministry and the Emergency Situations Ministry and the law-enforcement agencies, do everything possible to help people -- everything that you consider has not been done," Putin said. "And do whatever is necessary to restore [train] traffic as soon as possible."
The train was an overnight service along one of the country's busiest rail routes.
Some 800 meters of track was damaged by the blast.
There has been no indication of who may be responsible for planting the bomb.
However, Nikolai Patrushev, director of the Federal Security Service, linked the incident to violence in Russia's restive North Caucasus region.