UGLOVKA, Russia — Russian officials opened a terrorism investigation Saturday, saying that a homemade bomb planted on the tracks of the high-speed Moscow-to-St. Petersburg route caused a derailment that killed at least 26 people and injured dozens more.

The head of Russia's Federal Security Service, Alexander Borotnikov, was quoted by the Interfax and RIA Novosti news as saying that an improvised explosive device equivalent to 15 pounds of TNT had detonated when the train passed over it Friday night. Remains of the device were found at the site of the crash, Borotnikov said.

"Indeed, this was a terrorist attack," Interfax cited Vladimir Markin, a spokesman for federal prosecutors, as saying.

If terrorism is confirmed, the train derailment would be Russia's deadliest terrorist strike outside the volatile North Caucasus region in years. The high-end train was popular with government officials and Russian business executives.

President Dmitry Medvedev called for calm, saying "we need there to be no chaos, because the situation is tense as it is."