Suspected Islamic suicide bombers have killed 13 people and wounded scores more in the Russian republic of Dagestan on Friday, according to reports.
The two car bombs exploded outside a police station in Makhachkala, one at a police checkpoint and another 20 minutes later when the area was swarming with rescue workers, troops and firefighters.
Seven police officers and three firefighters were killed according to Reuters.
As a result of the second blast, 12 people died including seven policemen, three employees of the region emergencies ministry's rescue service and two local residents, the Moscow-based investigators said in a statement.
The second vehicle was deliberately timed to cause maximum casualties as rescue workers rushed to the scene of the first blast, investigators added.
Militants in the Caucasus have been battling the Kremlin to establish an Islamic state since the collapse of the Soviet Union.
Dagestan, one of the most lawless regions in the Caucasus area, has been subjected to frequent attacks by militants, although Friday's blast was the deadliest this year so far.
Moscow fought two vicious wars against separatists in Chechnya during the 1990's, but since then the movement has become increasingly Islamic and spread to other regions such as Ingushetia and Dagestan.