Russian news agencies say 22 people have been killed and 55 others injured when four cars of a Russian train travelling from Moscow to St Petersburg derailed on Friday.
One source has suggested the crash was caused by a bomb.
"A one metre-diameter hole has been found next to the railway track. Witnesses heard a loud slap before the accident. All of this could point to a possible act of terrorism," Interfax news agency quotes an unnamed source in Moscow's law enforcement agencies as saying.
The same line was hit by a bomb attack in August, 2007, although nobody was killed.
Islamist militants have sporadically attacked targets outside the north Caucasus region where there are a number of simmering conflicts surrounding Chechnya.
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