At least 15 law enforcement officers and four civilians were killed in two seemingly coordinated attacks by gunmen in Russia’s southern republic of Dagestan, Russian investigators said on Monday.

Wielding rifles and Molotov cocktails, the attackers assaulted synagogues and Orthodox churches on Sunday night in two major cities of Dagestan, a predominantly Muslim region on the Caspian Sea.

One of the civilians killed was Nikolai Kotelnikov, a priest in the city of Derbent. The attackers also set fire to a synagogue in the city.

For hours, the gunmen were on the loose, engaging in shooting sprees with members of the law enforcement, according to statements from the region’s interior ministry. Five attackers were eventually killed, local officials said.

Russian investigators designated the attack an act of terrorism, but it was not immediately clear who was responsible.

Local officials declared a three-day mourning period in Dagestan, a multireligious and ethnically diverse region, and said families of the victims would receive special compensation.