Russian authorities have identified the suicide bombers responsible for two attacks that killed 34 people in the city of Volgograd last month.
Police said on Thursday they have also and arrested two suspected accomplices in the violence-plagued Dagestan province.
The National Anti-Terrorism Committee identified the 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.
Two brothers, suspected of helping send the bombers to Volgograd, were also detained in Dagestan on Wednesday, the committee said. It identified them as Magomednabi and Tagir Batirov and said the investigation was continuing.
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 700km northeast of Sochi, where the Winter Olympics start next week.
Russia is mounting a massive security operation for the Olympics, deploying more than 50,000 police and soldiers amid threats from Islamic fighters.
A group from Russia's North Caucasus threatened last week to attack the Sochi Winter Olympics in a video published online.
The video featured what it said were the Volgograd bombers donning explosive belts and warning President Vladimir Putin to expect a "present" at the Olympics.