By Conor Humphries
MOSCOW (Reuters) - Islamist militants on Wednesday claimed responsibility for a bombing that derailed a Russian express train, killing 26 people, and vowed further "acts of sabotage" in a letter posted on a rebel website.
Friday night's attack on the luxury Nevsky Express running between Moscow and St Petersburg was the worst in Russia outside the North Caucasus in five years and raised fears of a new wave of bombings in major cities.
"This operation was prepared and carried out...pursuant to the orders of the Emir of the Caucasus Emirate Doku Umarov," the KavkazCenter.com website said, quoting a letter it said it received from Islamist rebels.
Umarov is Russia's most wanted guerrilla leader and leads a jihad insurgency which aims to free the mainly Muslim North Caucasus from Moscow's rule.
A second, smaller bomb detonated by mobile phone injured Russia's top detective as he visited the crime scene on Saturday. The Investigative Committee said its chief Alexander Bastrykin was hospitalised as a result of the blast but would not comment on his condition.
The militants' letter said the train bombing was part of a campaign of sabotage against strategic economic targets.
"These acts of sabotage will continue for as long as those occupying the Caucasus do not stop their policy of killing ordinary Muslims," the letter said.
Financial markets, hardened to violence in Russia, shrugged off the news. The RTS stock index was up 0.38 percent on the day by midday while the rouble held stable against the dollar.
Russia has been fighting a low-level Islamist insurgency in the North Caucasus republics of Dagestan and Ingushetia for more than a decade, following two wars in Chechnya to crush separatists.
Officials claim some success in pacifying Chechnya but the situation elsewhere has deteriorated, with regular shootings and bombings. President Dmitry Medvedev last month described the North Caucasus as Russia's biggest domestic problem.
On Monday, another bomb exploded under a train en route from Siberia to Azerbaijan in Russia's troubled Dagestan region but there were no deaths. Prime Minister Vladimir Putin described that blast as a "similar act of terror" to the Friday attack.
The KavkazCenter rebel website has in the past carried claims of responsibility for guerrilla attacks which law enforcement officials have subsequently disputed.
In August rebels used the site to claim responsibility for a Siberian dam disaster that killed over 70 people, but which authorities insisted was due to a technical fault.
Grigory Shvedov, the editor of news portal www.Caucasianknot.info, which tracks violence in the North Caucasus, said that the claim suggested Islamist militants carried out the attack but did not prove it.
"We can't be even be 90 pct sure that this claim of responsibility is giving us the right understanding of what happened," he said.
The theory that Islamists were behind the train bombing has gained momentum in recent days.
Top detective Bastrykin told state newspaper Rossiyskaya Gazeta that the attack bore hallmarks of Islamist rebels.
The follow-up explosion "is a tactic used by terrorists in the North Caucasus," he said. His spokesman declined to comment on Wednesday's claim of responsibility.
Interfax news agency on Wednesday quoted a source as saying that police were looking for four people from the Caucasus region who were seen in a nearby village ahead of the attack.
(Editing by Michael Stott and Jon Boyle)
