By Denis Sinyakov
UGLOVKA, Russia (Reuters) - A bomb caused a Russian train crash that killed dozens of people and injured 100 more, officials said on Saturday, stoking fears of an upsurge in attacks in Russia's heartland.
The 14-carriage Nevsky Express, with around 700 people on board, was jolted off the rails on Friday night on the main line between Moscow and Russia's second city, St Petersburg.
Three carriages of the luxury train, which is popular with officials and business executives, lay battered beside the rails after the blast.
It was the worst Russian bomb attack outside the mainly Muslim North Caucasus since a spate of suicide attacks in 2004.
"A bomb equivalent to 7 kg (15.4 lb) of TNT was detonated," the head of the Federal Security Service (FSB) domestic intelligence agency, Alexander Bortnikov, told Russian President Dmitry Medvedev, citing the results of a preliminary investigation.
No group has publicly claimed responsibility for the blast but a rise in bombings and suicide attacks over recent months in Ingushetia and Chechnya had raised concerns that the violence could spread deeper into Russia.
Analysts said such an escalation would likely turn the local Islamist insurgency in the North Caucasus into a major political issue for Russia's leaders.
Medvedev sent his condolences to the families of the dead and told ministers to ensure everyone received proper medical care and compensation. "You must make sure there is no chaos," Medvedev told ministers on a video conference.
Detectives said they had found fragments of a bomb at the scene of the derailment, near the village of Uglovka about 350 km (200 miles) north of Moscow, and opened a criminal case under terrorism laws. A 1-meter (3 ft) wide crater was visible under the rails, one of which was twisted and broken.
Some witnesses said they heard a loud bang, but another passenger told reporters in St Petersburg there had been no blast. An elderly woman who lives in a nearby log cabin said: "I thought it was an earthquake -- the ground shook."
Russia's Emergency Ministry said at least 26 people had been confirmed dead with another 18 missing, though one rescue official earlier put the death toll as high as 39. Prosecutors said more than 100 people had been injured.
During the early hours, conscripts in camouflage uniform carried bodies away from the scene through the surrounding woods. One body was wrapped in rags.
As hundreds of rescue workers toiled for hours, cutting through the tangled steel of wrecked carriages to search for survivors, railway officials said a second blast was detonated nearby on Saturday afternoon.
No one was injured by the second bomb, the chief of Russia's state railway operator, Vladimir Yakunin, said.
He said the attack showed many similarities to an explosion in August 2007 which derailed a similar Nevsky Express train on the same route, injuring 30 people.
Prosecutors at the time arrested two residents of the mainly Muslim North Caucasus region of Ingushetia, but said the mastermind behind the attack was ex-soldier Pavel Kosolapov, a former associate of late Chechen rebel commander Shamil Basayev.
The Interior Ministry said they had identified several suspects linked to Friday's attack, but gave no details about a possible motive.
"The so-called Chechen trace is traditionally viewed as the main one during investigations of such disasters," said Alexei Mukhin, an analyst at the Center for Political Information.
But Mukhin added that outdated Soviet-era infrastructure was often the cause of major accidents in Russia.
The derailment has delayed about 27,000 people as transport officials try to divert trains onto smaller lines, railway officials said.
Friday's railway disaster was the deadliest since December 2003 when a bomb blast tore through a passenger train in the North Caucasus, killing 47 people.
(Additional reporting by Maria Kisyelova, Gleb Stolyarov and
Dmitry Zhdannikov in Moscow and Denis Pinchuk in St Petersburg; Writing by Guy Faulconbridge and Conor Humphries; Editing by Mark Trevelyan)