Police released a composite sketch Monday of a man thought to be involved in the bombing of a Moscow-to-St. Petersburg train that killed 26 people, and a railroad official suggested the attack had the hallmarks of terrorism by insurgents from volatile North Caucasus.

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.

Many police in the North Caucasus have been killed in blasts to have ripped through the site of an initial explosion that they had arrived to investigate.

The Moscow-St.Petersburg train line is very popular with Russian business executives and government officials

Health Minister Tatyana Golikova said Monday a woman injured in the derailment died late Sunday in a Moscow clinic, bringing the death toll to 26, three Russian news agencies reported. Dozens of people were treated at hospitals for their wounds, some flown into Moscow and St. Petersburg by helicopter.

Russia mourned the train victims Monday, with many entertainment events postponed or canceled.

No suspects or motive have been named, but police released a computerized sketch Monday of a possible suspect. It was not clear, however, if the black-and-white composite depicted the man whom Interior Minister Rashid Nurgaliyev had spoken of earlier, a man about 40 years old with red hair.

The business daily Kommersant cited an unidentified police source as saying authorities suspect the latest bombing involved the same criminal group linked to an almost identical attack on the same track in 2007. The 2007 attack injured dozens in the train that passed over an explosive device; the motive went unexplained.

Two suspects in the 2007 blast were detained but a third, Pavel Kosolapov, a former military officer believed to have links to Chechen separatists, remains a fugitive.

Meanwhile, a small explosion early Monday damaged a section of railroad track in the volatile North Caucasus republic of Dagestan. There were no injuries and a train passing at the time was unaffected, local transport police spokesman Akhmed Magomayev told The Associated Press.