Russian investigators are probing a Chechen trail in the bomb blast that derailed a Moscow-St. Petersburg train on Friday night.

The attack killed 25 people but the death toll may go much higher as 26 out of 660 passengers who travelled on the Nevsky Express remain missing. Rescue workers retrieved many fragments of mangled bodies from the last three carriages that flew off the track. Over 100 people were injured, including 21 in grave condition, said Health Minister Tatyana Golikova on Sunday.

A second explosive device partially detonated at the crash site later on Saturday as railway workers were clearing the debris.

A Russian racist group, Combat-18, claimed responsibility for Friday’s blast, but security sources said it was more likely the derailment had been masterminded by the same man who blew up another Nevsky Express in the same area two years ago. The 2007 attack was blamed on ex-soldier Pavel Kosolapov, an ethnic Russian who adopted Islam and joined Chechen rebels in the 90s. Prosecutors have arrested two residents of the North Caucasus in connection with the bombing, but have so far failed to catch Kosolapov.

A close associate of the late Chechen warlord Shamil Basayev and head of Riyadus-Salikhin, a Chechen suicide squad, Kosolapov is believed to have staged a string of terrorist attacks in Russian cities in the 2000s, including a deadly bombing in the Moscow metro in 2004 that killed 42 people and wounded over 250. Riyadus-Salikhin was disbanded after Basayev’s death, but Chechen rebel leader Doku Umarov recreated the group earlier this year.

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