Russia has arrested a police colonel on terrorism charges over the 2010 Moscow metro bombings.
Gazi Isaev, the head of the Russian Interior Ministry for the Kizlyar District of Dagestan, was detained on Monday, according to a statement by Russia's Investigative Committee.
The police colonel is accused of directly aiding a suicide bomber who detonated a device at one of two city centre metro stations at rush hour on 29 March 2010.
40 people were killed in the bombings and more than 100 others were injured in the explosions at Lubyanka and Park Kultury metro stations.
Russian authorities denounced the bombings as "the deadliest and most sophisticated terrorist attack in the Russian capital in six years".
The Investigate Committee say that Gazi Isaev was a member of the banned terrorist organization "Imarat Kavkaz" ("Caucasus Emirate") between 2009 and 2010.
He is said to have used his official position to provided information to the group's leaders about police activity in Kizlyar and special operations against illegal armed groups.
Isaev is also accused of personally transporting senior members of the group across the southern region of Dagestan.
He is alleged to have driven one of the female suicide bombers with an improvised explosive device to a bus station in the Kizlyar area, from where she would travel 1,470 km north to Moscow to carry out the attack.
The investigation is being carried out in cooperation with Russia's Federal Security Service (FSB).
The man thought to have masterminded the twin suicide bomb attacks was killed during an operation by security forces in Dagestan in August 2018.