Russian security officials say they suspect the derailment of a train traveling from Chechnya to Moscow Sunday morning was an act of terrorism. Several people were injured in the early-morning incident.
Rescuers initially reported a technical malfunction was suspected in the derailment, about 150 kilometers south of Moscow.
But a closer inspection of the site led officials to say the incident appeared to have been deliberate.
Internal security officials say the train's engineer reported hearing an explosion just seconds before four train wagons jumped the tracks.
Federal Security Agency spokesman Nikolai Zakarov says a crater one-meter-wide was found next to the tracks, with wires leading from it. He says a criminal investigation has been launched.
The derailment occurred as the train was heading to Moscow from Grozny, capital of the troubled region of Chechnya.
Russian troops have been battling separatist fighters there for over a decade, and train service between Grozny and Moscow was restored only last year, after a five-year break due to the conflict.
The pro-Russian president of Chechnya blamed the incident on the rebels, who were held responsible for at least two previous train explosions that caused many deaths and injuries.
Sunday is a national holiday, known as the Day of Russia, when the country honors the first post-Soviet constitution that was written and adopted in the early 1990s.
The derailment took place just hours before President Putin held a Kremlin reception, in which he praised the constitution as one of the most democratic in the world.
Previous attacks by Chechen rebels have taken place on holidays, including the deadly siege at a school in southern Russia, marking the first day of classes last September 1. Over 300 people died in the three-day siege, half of them children.