Authorities say Friday night's derailment of a train speeding from Moscow to St. Petersburg was caused by a bomb planted on the tracks. The blast gouged out a five-foot crater and sent the final three carriages of the 14-car Nevsky Express hurtling off the rails. A second explosive device partially detonated at the crash site Saturday as railway workers were clearing debris.
Terrorism has been a major concern in Russia since the 1991 collapse of the Soviet Union, as Chechen rebels have clashed with government forces in two wars and Islamist separatists continue to target law enforcement officials.
A high-level official of the state-run railroad company official suggested militants from North Caucasus were likely behind the attack, citing the second explosion that he said was a hallmark of the insurgents.
"The second explosion, which occurred some time later, is the so-called double-blast method, which is carried out by North Caucasus sabotage groups," Alexander Bobreshov, a vice-president of Russian Railways, said on Ekho Moskvy radio.