MOSCOW (Reuters) - Russia has identified two suicide bombers responsible for attacks that killed 34 people in the city of Volgograd last month and arrested two suspected accomplices in the violence-torn Dagestan province, officials said on Thursday.
The National Anti-Terrorism Committee said the bombers, whose attacks raised fears of further violence before the Sochi Winter Olympics next week, were members of a militant group in Dagestan in the restive North Caucasus in southern Russia.
A bombing at the railway station in Volgograd on December 29 was followed a day later by a blast that ripped apart a trolleybus in the city 700 km (400 miles) northeast of Sochi, where the Olympics start on February 7.
The blasts were the deadliest attacks in Russia outside the North Caucasus, the cradle of an Islamist insurgency whose leader has urged fighters to prevent the Olympics going ahead, since a bomber killed 37 people at a Moscow airport in 2011.
The head of security for the Olympics issued Russia's latest assurance about security at the Sochi Games, a major prestige project for President Vladimir Putin, saying there was "no concrete threat".
The Anti-Terrorism Committee identified the Volgograd bombers as Asker Samedov and Suleiman Magomedov, called them members of the "Buinaksk Terrorist Group" and said it had known their names for some time. Buinaksk is a city in Dagestan.
Two brothers suspected of helping send the bombers to Volgograd were detained in Dagestan on Wednesday, the committee also said. It identified them as Magomednabi and Tagir Batirov and said the investigation was continuing.
Investigators initially said a woman was suspected of carrying out the railway station bombing.
A video posted on the Internet last week by a group identifying itself as Vilayat Dagestan featured what it said were the Volgograd bombers donning explosive belts and warning Putin to expect a "present" at the Olympics.
The video named the men only as Suleiman and Abdulrakhman, and it was not immediately clear if either of them was one of the men identified by the National Anti-Terrorism Committee as the suicide bombers.
Buinaksk is the site of the first of four deadly apartment building bombings in September 1999 that were a catalyst of Russia's second post-Soviet war against separatists in Chechnya, adjacent to Dagestan.
As prime minister at the time, Putin headed up the war, which helped shape his image as a strong leader and ensure his election the following year after Boris Yeltsin resigned and made him acting president.
But Putin, in power as president or prime minister ever since, has been unable to end attacks by Islamist militants from Dagestan and other mostly Muslim provinces of the North Caucasus, where the insurgency stems from the wars in Chechnya.
The leader of the insurgency, Chechen militant Doku Umarov, called on insurgents in a web-posted video in July to use "maximum force" to prevent the Olympics, a huge undertaking that will help shape Putin's legacy, from being held.
Putin and other Russian officials have assured athletes and visitors that they will be safe in Sochi.
"Not only our information, but the information of our partners ... shows that at the current time, there is no concrete threat to the Games," Alexei Lavrishchev, a senior Federal Security Service officer, said in Sochi on Thursday.
Asked about Dagestan during a news conference, he said it is "pretty far way" from Sochi and added: "The current situation in Dagestan is under control."
Dagestan, on the Caspian Sea about 570 km (350 miles) southeast of Sochi, is beset by near-daily violence linked to the insurgency.
Additional reporting by Piotr Pilat in Sochi; Editing by Mark Heinrich