Five suspects have been detained in connection with attacks in Kazan last week on two of Tatarstan's most prominent Muslim leaders, officials said.
Law enforcement officials also announced that the motive in at least one attack appeared to be related to the business interests of the targeted cleric, Chief Mufti Ildus Faizov, as well as "ideological disagreements."
Faizov was severely injured by a bomb blast inside his SUV on Thursday.
Thirty minutes before that attack, gunmen shot his former aide and prominent Muslim cleric Valiulla Yakupov as he was leaving his house.
He managed to reach his car, where he died from his injuries.
Working together, the Federal Security Service and police arrested the Muslim head of the Vakf parish, 39-year-old Kazan resident Murat Galeyev; 41-year-old Airat Shakirov, a resident of Tatarstan's Vysokogorsky district; Abdunozim Ataboyev, 26, an Uzbek native registered in Kazan; and a 36-year-old Kazan resident whose name was not disclosed.
The suspected mastermind of the attacks — the board chairman of the Idel-Hajj company, 57-year-old Rustem Gataullin — was also detained.
A Kazan court is scheduled to decide Monday whether to sanction his arrest.
One more man, Azat Gainutdinov, a 31-year-old resident of the republic's Laishevsky district, was detained Friday and later released for lack of evidence, the Investigative Committee said in a statement posted on its website.
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 pilgrimages to Mecca, the statement said.
Faizov and Gataullin came into conflict over Faizov's role in the firm, and Gataullin threatened Faizov, the statement said.
Gataullin also has links to radical Wahhabis, Kommersant reported.
Faizov and Yakupov were seen as anti-Wahhabi, and Tatarstan President Rustam Minnikhanov on Thursday indirectly blamed Islamic militants for the attacks.
Investigators did not comment on how Faizov's business dealings might relate to the attack on Yakupov, whose funeral was held in Kazan on Friday.
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.
President Vladimir Putin said Friday that he had discussed the latest attacks with the chief of the Federal Security Service.
"Law enforcement will do its best to find, expose and punish the criminals. I don't doubt it," Putin said, Interfax reported.