MOSCOW (AP) – A bomb packed with TNT blew a passenger train traveling from Chechnya to Moscow off its tracks Sunday, injuring at least 15 people on a Russian national holiday.

The blast occurred just hours before President Vladimir Putin marked the Day of Russia holiday with a reception and awards ceremony in the Kremlin.

Officials did not name any suspects and no group claimed responsibility. But Chechen separatist rebels frequently stage attacks on Russian holidays. Deputy prosecutor general Nikolai Savchenko said they had opened a criminal case on suspicion of terrorism and attempted murder, the Interfax news agency reported.

Putin made no public mention of the derailment, but police stepped up security at railroad stations and on trains themselves.

The train was traveling from the Chechen capital, Grozny, when the driver saw an explosion on the railbed ahead of the engine, Federal Security Service spokeswoman Diana Shemyakina said. Four of the cars jumped the tracks. NTV television said the train derailed shortly after leaving a station and was not moving fast, which may have saved lives.

“This hadn’t happened before … and I never thought it would happen,” a conductor on one of the cars that derailed, Aishat Iskhazhiyeva, told NTV, her voice trembling. “It was just one moment – we were powerfully shaken and I could hardly get to the emergency brake. I thought something happened under the train.”

FSB spokesman Nikolai Zakharov said they believe the blast was caused by a bomb containing more than six pounds of TNT. Investigators found a crater about 3 feet wide on the tracks at the site of the blast, about 90 miles south of Moscow. They also found wires attached to the right rail and a spot where the person who set off the bomb might have been.

Trains only started traveling the 1,000 miles between Moscow and Grozny a year ago after a five-year break due to the latest of two wars in the province in the past decade. Revival of the route was seen as part of a government effort to portray life in Chechnya as returning to a semblance of normality.