Russia's domestic intelligence service on Saturday said a train disaster that killed dozens and injured nearly 100 was caused by a bomb.
The Nevsky Express, carrying 682 passengers and 29 crew, was derailed on Friday night on the main line between Moscow and Russia's second city, St Petersburg.
"Criminal experts say that based on preliminary findings a bomb equivalent to 7kg of TNT was detonated," Alexander Bortnikov, the head of the FSB domestic intelligence service, told President Dmitry Medvedev in a televised meeting.
Russian detectives said they had found fragments of what they believed was a bomb and said they had opened a criminal case on charges of terrorism. No one has so far publicly claimed responsibility for the suspected attack.
The derailment is Russia's most deadly train disaster for years and could raise fears of a surge in attacks on the Russian heartland by rebels from the turbulent North Caucasus.
Russia's Emergency Ministry said at least 26 people had been confirmed as dead with another 18 missing, though one rescue official earlier put the death toll as high as 39. The ministry said 96 people had been injured.
The Nevsky Express was derailed at 9:34 p.m. (1834 GMT) on Friday near the village of Uglovka about 350 km (200 miles) north of Moscow.
A Reuters photographer saw soldiers carrying four body bags away from the scene where rescue workers cut through the tangled steel to search for survivors in two wrecked train carriages.