"Indeed, this was a terrorist attack," Interfax cited Vladimir Markin, a spokesman for federal prosecutors, as saying.
If terrorism is confirmed, the train derailment would be Russia's deadliest terrorist strike outside the volatile North Caucasus region in years.
Witness accounts appeared to back up reports of a bomb blast.
"It was immensely scary. I think it was an act of terrorism because there was a bang," passenger Vitaly Rafikov told Channel One state television. He was unhurt in the accident and helped with the rescue, hauling victims from the wreckage and lighting fires for warmth.
The last three carriages of the 14-car Nevsky Express careered off the tracks Friday night as the train approached speeds of 200 kilometers per hour (130 mph), officials said.
More than 600 passengers were on the train when it derailed near the border of the Novgorod and Tver provinces. The rural area is 250 miles (402 kilometers) northwest of Moscow and 150 miles (250 kilometers) southeast of St. Petersburg.
