A remote-control explosive device was detonated late Thursday in the capital of Daghestan, Makhachkala, damaging a bus that was carrying a group of law enforcement officers.
A platoon commander and three policemen were hospitalized with contusions. A civilian passerby was also caught in the blast, sustaining minor injuries.
The explosion left a 40-cm (15-inch) crater. Experts believe its power was equivalent to 4 kg of TNT.
The source also said another bomb, which apparently failed to explode, was found at the site hidden in a bucket.
Sporadic terrorist attacks and militant clashes are common in the Russian republics of Ingushetia and Daghestan, and in neighboring Chechnya, although the Kremlin has officially ended its military campaign to fight separatists and terrorists there.
