Islamist militants claimed responsibility on Wednesday for a bombing that derailed a Russian express train, killing 26 (28-according to different sources) people, according to the website KavkazCenter.com.
The attack was carried out on orders of "the Emir of the Caucasus Emirate Doku Umarov," the website said, quoting a letter it said it received from Islamist rebels.
Below are some facts about Umarov and his rebels:
* Umarov, a veteran field commander, poses himself as president of the "Chechen Republic of Ichkeria" -- the term separatists use for a Chechen territory independent of Russia. He took the role in 2006 after Chechen rebel leader Abdul-Khalim Sadulayev was killed in a battle with Russian troops.
* Rebel websites say Umarov leads small units of 10 to 12 men each operating in the Chechen mountains and waging guerrilla warfare against Russian special forces.
* Umarov participated in both Chechen wars in the 1990s, fighting Russian security forces.
* Wounded several times in combat with Russian forces, Umarov has scars on his lips and chin and walks with a limp, according to a RFE correspondent who interviewed him in 2005
* In that rare interview, Umarov said that only financial restrictions prevented him from carrying out more attacks and vowed to continue his guerrilla war. "As long as we have not completely liberated ourselves from the boots of Russian soldiers, I do not see any other way out", he told RFE.
* Russia describes Umarov as a "terrorist" and its forces have hunted him for years. He is Russia's most wanted guerrilla leader.
* "Umarov was for many years the second person in the terrorist team and during recent years he became primarily the main person although he wasn't ever seen as the organiser of terror activities," said Grigori Shvedov, the editor of www.caucasianknot.info
* In June, Chechen President Ramzan Kadyrov said Umarov was badly wounded in a gun battle, after initial reports said he had been killed.
* Umarov's fighters claimed responsibility for an Aug. 17 disaster which wrecked Russia's biggest hydroelectric dam in Siberia and killed 75. Authorities strongly denied the claim and insisted that the disaster was an industrial accident.
* Umarov's group said at the time that fighters had been sent across Russia for attacks that would focus on gas and oil pipelines, power plants and electricity lines.
* Umarov also claimed responsibility for the June 22 suicide bomb attack on Ingush president and Kremlin appointee Yunus-Bek Yekurov. Yevkurov narrowly survived that attack, one of a growing number of major incidents across Russia's North Caucasus republics of Ingushetia, Dagestan and Chechnya.
* Chechen guerrillas, then run by warlord Shamil Basayev, who died in 2006, carried out the 2004 Beslan school siege in southern Russia in which more than 320 people died. Most of the victims were children.
Reuters has contributed to the report.