Russian investigators have searched for more evidence and drawn up and distributed to police composite sketches of two suspects in the bombing that sent an express passenger train hurtling off the tracks, injuring 60 people and raising fears of terrorist attacks before crucial elections.
The head of Russia's main security agency, meanwhile, suggested that terror threats were still acute and that police agencies were prepared to bolster security and surveillance nationwide before parliamentary and presidential elections to be held in December and March.
"The threat of terrorism and extremism has not yet been eliminated," Federal Security Service director Nikolai Patrushev said on Tuesday at a meeting of the National Anti-Terrorism Committee -- a grouping of senior law enforcement officers, ministers and agency heads.
"These events again are evidence that the [committee] made the timely decision to examine the question of taking anti-terrorist security measures" before the elections, Patrushev said in televised comments.
Prosecutors opened a terrorism investigation into the blast that hit the Neva Express train late on Monday, as it was traveling at 191kph from Moscow to St Petersburg. The locomotive and a dozen passenger cars derailed near the city of Novgorod, about 500km north of the capital.
In all, 25 people were hospitalized after the derailment, and 35 others sought treatment at the scene, Russian Railways said in a statement.
The bomb was placed about 30m ahead of where the tracks cross a bridge over a roadway. But the train crossed the bridge before going off the tracks; the casualty toll likely would have been significantly higher if the train had gone off the approximately 20m high bridge.
There were no reported claims of responsibility, but the incident comes as violence has grown markedly in recent months in the restive North Caucasus region where Chechnya is located.
The Interfax news agency, citing unnamed sources, said cables and other evidence found at the blast site strongly resembled the equipment used to detonate a bomb under a train heading from the Chechen capital, Grozny, to Moscow in 2005. That blast injured 42 people.
The incident also recalled the bombings of two trains in southern Russia in recent years. A bomb placed in a baggage car in a train heading from Kislovodsk to Mineralny Vody killed 47 people on Dec. 5, 2003. Less than three weeks later, a bomb was detonated under a locomotive of a freight train in Chechnya.
Insurgents from Chechnya and North Caucasus or their sympathizers were suspected of involvement in those blasts.
Transportation police officials told Russian news agencies that composite sketches of two men compiled from witness accounts of people seen hanging around in the area had been sent out to law enforcement agencies.
The Moscow-St Petersburg route is one of the busiest for the Russian Railways network, and is popular particularly among foreign tourists. There were no reports of foreigners being injured.
The blast and derailment tore up about 800m of track.