"A criminal case has been opened under article 205, clause 3, which is terrorism."
The derailment occurred around 9:30pm (1730 GMT) near Malaya Vishera in Novgorod province, about 500km northwest of Moscow.
The train's driver heard a "loud bang" from underneath the locomotive immediately before the incident happened, Alexander Pirkov, an adviser to Russian Railways' senior executive, said in televised comments late on Monday.
A local police source told RIA Novosti news agency that "a bomb equivalent to two kilogrammes of TNT exploded on the rails".
Jonah Hull, Al Jazeera's correspondent in Moscow, said: "There are growing fears here that this was an act of organised terrorism. To that end, the deputy prosecutor-general spoke to journalists at the scene earlier today and pointed out that terrorism was just one of a number of possibilities being considered.
"They'll need to be looking at the possibility, also, that if this wasn't an organised act of terrorism, it may have been some private act of criminality."
There was no indication of who may have planted the bomb, but groups fighting against the Russian government in Chechnya have carried out attacks in the past.
The Interfax news agency, citing an unnamed law-enforcement official, said investigators had found a crater under the train's ninth carriage.
|"We heard two explosions, then the |
train put on
the brakes suddenly"
A conductor on the train showed Reuters news agency a video on his telephone of a crater about two metres across on a bridge the train had just crossed.
"We heard two explosions, then the train put on the brakes suddenly," one conductor, who did not want to be identified, said.
"The train shook. A panic started.
"We smashed out the glass and helped the passengers out ... The worst damage was in the restaurant car. That is where most of the casualties were."