Law enforcement officials also announced that the motive for the crime appeared to be related to business interests of at least one of the attacked leaders, Chief Mufti Ildus Faizov, as well as "ideological disagreements."

On Thursday, Faizov was severely injured by a bomb blast that went off inside his SUV. Thirty minutes before the attack on him, gunmen shot prominent Muslim cleric Vaiulla Yakupov while he was leaving his house in the capital of the Tatarstan republic. He managed to reach his car, where he died from his injuries. 

Four suspects have been detained in connection with the attacks: head of the board of directors of the company Idel-Hajj, 57-year-old Rustem Gataullin; head of the Vakf parish, 39-year-old Kazan resident Murat Galleyev; 41-year-old Airat Shakirov, a resident of Tatarstan's Vysokogorsky district; and 31-year-old Azat Gainutdinov, a resident of the republic's Laishevsky district, the Investigative Committee said in a statement posted on its website.

The statement noted that after Faizov was elected chief mufti of Tatarstan, in 2011, he took over management of the financial operations of Idel-Hajj, which assists Muslims on pilgrimage trips to Mekka.

Faizov and the head of Idel-Hajj came into conflict over Faizov's role in the firm, and the head of the company threatened Faizov, the statement said.

Investigators did not describe how the motive connected to Faizov's business dealings could relate to the attack on Yakupov.

Observers have noted that both Faizov and Yakupov are seen as anti-Wahhabi, and Tatarstan President Rustam Minnikhanov on Thursday indirectly blamed Islamic militants for the attacks.

Thursday's attacks represented the first major assault on Muslim leaders in Russia outside the North Caucasus, where a string of moderate religious clerics have been killed over the past two years.