A homemade chemical bomb planted on the tracks in an apparent act of terrorism derailed a high-speed train in Russia carrying hundreds of people Saturday, killing at least 26 and injuring scores more.

The head of the Russian Federal Security Service told the country's news agency Interfax that traces of explosives were found at the train crash site, including chemical residue from a homemade bomb.

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 express train traveling from Moscow to St. Petersburg passed over it Friday night about 9:30 p.m.

"There is little doubt this is terrorism," a source in Russian Prime Minister Vladimir Putin's office told Fox News. Who was responsible for the derailment or why it occurred "remains unclear," the source said.

A second explosive device partially detonated Saturday during the clear-up operation near the disaster site, according to the head of Russian Railways, Vladimir Yakunin.

Remains of the device were discovered at the scene, Borotnikov said. Twenty-six people were killed in the accident and nearly 100 injured after the last three cars of the 14-car Nevsky Express went off the tracks in the Tver province northwest of Moscow.

"Indeed, this was a terrorist attack," Interfax cited Vladimir Markin, a spokesman for federal prosecutors, as saying. He told the ITAR-Tass news agency that the bomb crater on the track was 5 feet deep.

Russian Railways President Vladimir Yakunin told reporters Saturday that the Friday accident may have been caused by an explosion under the tracks.

The derailment of the upscale train, which was popular with government officials and business executives, would be Russia's deadliest terrorist strike outside the volatile North Caucasus region in years.

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. He said he helped with the rescue, hauling victims from the wreckage and lighting fires for warmth.

Passenger Igor Pechnikov was 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 light rain started to fall at the scene of the derailment at daybreak Saturday as emergency workers huddled around fires, wrapped in blankets, and two huge cranes lifted pieces of the wreckage clear of the site as crews continued the search for victims.

One of the bashed and battered railway carriages lay on its side across the tracks, while baggage and metal debris lay scattered in the muddy ground.

Police and prosecutors swarmed over the site and restricted access to what was reported to be a bomb crater.

Friday night's Nevsky Express was carrying 633 passengers and 20 railway personnel during its regular run to St. Petersburg, the emergencies ministry said.

The derailment occurred in a rural area near the border between the Novgorod and Tver provinces, about 250 miles northwest of Moscow and 150 miles southeast of St. Petersburg, authorities said.

Russian trains have been the targets of bombers in the past.

An explosion on the Moscow-St. Petersburg line in 2007 derailed a passenger train and injured 27 people. Two suspects have been arrested and authorities are searching for a former military officer they believe was behind the blast, but the motive was unclear.

A December 2003 suicide bomb attack on a commuter train near the Russian republic of Chechnya killed 44. At least 12 people were injured in June 2005 when a bomb derailed a train headed from Chechnya to Moscow.

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.

But Russia has also been plagued by deadly accidents resulting from its deteriorating Soviet-era infrastructure, a high incidence of alcohol abuse and from negligence.

Russian news agencies reported that some injured passengers were taken by train and bus to hospitals in the area and to St. Petersburg for medical attention.

State-run Vesti-24 broadcast live from the national crisis response center in Moscow. Medvedev ordered authorities to help the victims and determine what caused the derailment, state media cited the Kremlin as saying.

Fox News' Dana Lewis and The Associated Press contributed to this report.