Moscow ? A bomb apparently wired to railroad tracks derailed a passenger train traveling from Chechnya to Moscow on Sunday, injuring at least 15 people, two of them seriously, 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 examine the scene of a derailed train about 90 miles south of Moscow. The train traveling from Grozny, Chechnya, to Moscow on Sunday was derailed by an explosion on the tracks, the Federal Security Service said.

Officials did not name any suspects, and no group claimed responsibility. But Chechen separatist rebels frequently stage attacks on Russian holidays.

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

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.

FSB spokesman Nikolai Zakharov said they believed 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 resumed service on 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 normal.

The train from Grozny to Moscow, which takes two days to make the trip, runs twice a week.