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 7 kilograms 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." Witness accounts appeared to back up reports of a bomb blast. "It was immensely scary. I think it was an act of terrorism because there was a bang," passenger Vitaly Rafikov told Channel One state television. Uninjured in the accident, he helped with the rescue, hauling victims from the wreckage and lighting fires for warmth. The last three carriages of the 14-car Nevsky Express careered off the tracks Friday night as the train approached speeds of 200 kilometers per hour (130 mph), officials said. More than 600 passengers were on the train when it derailed near the border of the Novgorod and Tver provinces. The rural area is 250 miles (402 kilometers) northwest of Moscow and 150 miles (250 kilometers) southeast of St. Petersburg. Reports on the death toll varied. Health Minister Tatyana Golikova said at least 26 people were killed, 18 were missing and nearly 100 were injured and hospitalized in the derailment. The Prosecutor General's office said the death toll had risen to 30, with 60 others in the hospital. Police and prosecutors swarmed over the disaster site Saturday and restricted access to what was reported to be a bomb crater. Rescue workers scoured the wreckage, searching for the missing, as two huge cranes lifted up pieces of twisted metal. Their efforts were hampered later Saturday when a small explosion was heard, forcing Russia's security services to close rail links between the two main cities that had been partially reopened, Interfax reported. There was no elaboration. Passenger Igor Pechnikov described being in the second of the three derailed cars. "A trembling began, and the carriage jolted violently to the left. I flew through half of the carriage," he said. A battered railway carriage lay on its side across the tracks Saturday, while baggage and metal debris were scattered in the mud. Emergency workers wrapped up in blankets and huddled around fires as a light rain started to fall. Terrorism has been a major concern in Russia since the 1991 collapse of the Soviet Union, as Chechen rebels have clashed with government forces in two wars and Islamist separatists continue to target law enforcement officials. If terrorism is confirmed, it would not be the first time the Moscow-St. Petersburg rail line has been attacked. A 2007 derailment on the line was caused by an explosion and injured 27 people. Authorities arrested two suspects and are searching for a third - a former military officer. Across Russia's North Caucasus region, attacks are relatively frequent. In August, a suicide bombing of a police station in Ingushetia's capital killed 25 people and injured 164. A September 2004 attack on a school in the North Ossetian town of Beslan ignited a three-day hostage-taking ordeal in which more than 330 hostages were killed in a botched rescue. In addition, a December 2003 suicide bombing of a train near Chechnya killed 44 people. But outside the volatile southern region, the last fatal terrorist attacks occurred in August 2004. A suicide car bombing in Moscow that month killed 10 people only days after bombs ripped through two passenger aircraft, killing more than 80 people. Those attacks were blamed on Chechen rebels, as was a February 2004 Moscow subway bombing that killed 40 people. A 2002 hostage-taking at a Moscow theater ended with the deaths of around 130 people. Another train derailment in June 2005 left at least 12 injured on a train that had been traveling from Chechnya to Moscow.