* Volley of deadly bombings in Volgograd kills 41 in three months
* Insurgency leader Umarov vows to attack Winter Games
* Involvement of ethnic Russians in attacks marks new strategy
By Alissa de Carbonnel
MOSCOW, Jan 1 (Reuters) - The suspected involvement of converts to Islam in Russian suicide bombings points to the growing reach of jihadists far beyond the Muslim provinces of Chechnya and Dagestan, where insurgency and separatism have simmered for two decades.
Russian news media say the authorities suspect an ethnic-Russian convert to Islam may have been behind one of the two suicide bombings that killed a total of 34 people in the past two days in Volgograd, a southern Russian city.
Another convert is suspected of building a bomb used to kill seven people in the same city two months ago.
The attacks came half a year after two Chechen brothers who had lived in Dagestan became the suspects in the Boston Marathon bombings that killed three Americans, sign that a conflict once seen as remote by the West could have consequences far afield.
Security experts say that insurgents have used ethnic Russians to carry out attacks in other parts of Russia, both because of the symbolism of their conversion to radical Islam and because Slavic appearance could help them avoid detection.
"This is a new strategy that we have been seeing more often lately. It's a massive problem for law enforcement agencies," said Andrei Soldatov, an expert on Russia's security services.
Pavel Pechyonkin, named by Russian news agencies as a possible suspect in the first of two attacks within 24 hours - a suicide bomb that killed 18 people at Volgograd's railway station on Sunday - was a paramedic from the Mari El region in central Russia.
An ethnic Russian on his father's side, he converted to Islam, his mother's religion.
He left home in 2011 to join insurgents in Dagestan, his parents said earlier this year in a video message posted on the Internet, appealing to their son to lay down arms.
In response, Pechyonkin recorded his own video message, saying he was following God's will.
"Here Muslims are being killed and kidnapped ... Why should we follow those Christian commandments, when Allah urges us to fight those kafirs? Why shouldn't we leave their children orphaned?" he said, wearing a green tunic and skull cap.
Authorities also believe an ethnic Russian from the Moscow suburbs, Dmitry Sokolov, built a suicide explosive belt detonated by his Dagestani wife in a bus bombing in Volgograd in October, law enforcement sources in Dagestan said.
The two met on online Islamist chat rooms. Sokolov was killed by Russian security forces in November, alongside four other militants in a house in Dagestan.
RECRUITS FROM AS FAR AS CANADA
Vladimir Putin crushed separatists in Chechnya when he rose to power 14 years ago. But an Islamist insurgency spread to neighbouring Dagestan and remains the deadliest conflict in Europe. Fighters have recruited to their ranks from as far afield as Canada.
Yekaterina Sokirianskaya, a Caucasus expert at International Crisis Group, says many new converts adopt a fundamentalist form of Islam that often puts them in conflict with their families and makes them more prone to "radicalisation".
"They are very attractive to insurgents," Sokirianskaya said. "The last attack could have been carried out only by a Slavic man, this is clear, because security measures were tightened and a women in a hijab would have been noticed."
Heavy security around Sochi means an attack on the Black Sea resort city where the Olympics will be held in February would be extremely difficult, security experts say, but the greatest potential threat is from a suicide bomber.
"This is very effective tactic. It requires very little preparation and very little money, but it is very hard to stop," Alexei Filatov, deputy head of the veterans' association of the Alfa anti-terrorism unit.
Russian police have launched a security operation making no secret that they are targeting migrants from Muslim areas. A bomber recruited from another part of Russia, preferably with a Russian-sounding name, would have an easier time reaching a target than one with a Muslim name whose identity documents were issued in Chechnya or Dagestan.
More than 120 people have become suicide bombers during Putin's rule, Grigory Shvedov, editor of website Kavkaz-uzel.ru, which tracks the unrest.
A harsh crackdown on adherents of the strict Salafist strand of Islam practiced by militants has added fuel to the insurgency, Shvedov and other experts say.
"Although brute force is being used in the North Caucasus now, they (the authorities) cannot build a wall thick enough to prevent terrorists from slipping out," Shvedov said.
Local militant groups in Chechnya, Dagestan and other North Caucasus provinces united in 2007 under the leadership of Doku Umarov, a former Chechen rebel, whose Caucasus Emirate group says it was behind suicide bombings that killed 37 people at a Moscow airport in 2011 and 40 on the Moscow subway in 2010.
He urged his fighters in a video posted online in July to use "maximum force" to prevent Putin staging the Olympics.
Volgograd, about a day's drive north along a main highway from the Caucasus, is a far easier target for militants than Sochi, the site of the Olympics, 700 km (440 miles) away.
Sochi, a 145-km (90-mile) long stretch of coastal resorts where Putin himself spends his summer holidays, has had its security beefed up with forces drawn from other cities. It is shielded by impassable mountains on one side and the Black Sea on the other, and can only be approached by air or a heavily guarded coastal road.
If they cannot reach Sochi itself, militants may have calculated instead on the symbolic value of sowing panic in Volgograd, one of the biggest cities in southern Russia with more than 1 million people.
The attack subverts its image as bastion of Russian strength earned through victory in a decisive battle in World War Two, when the city was known as Stalingrad.
"A symbol of Russia's tragedy and triumph in World War Two has been singled out by the terrorists precisely because of its status in people's minds," said Dmitry Trenin, the director of the Moscow Carnegie Centre. (Additional reporting by Alexei Anischuk and Gabriela Baczynska in Moscow and Maria Tsvetkova in Volgograd; Editing by Peter Graff)