Russian Investigative Committee spokesman, Vladimir Markin, said Friday police detained the four people, in the death of Valiulla Yakupov, who was shot as he left home in Kazan, and died in a car on the way to a hospital. Yakupov had served as the top deputy to Ildus Faizov, the mufti of Tatarstan, who was injured in a separate car bombing, ITAR-Tass reported.
Yakupov was recently put in charge of religious education for the region's spiritual directorate.
Investigators said the separate attacks were possible reprisals for the two clerics' criticism of radical Islamists, The Wall Street Journal reported.
The Thursday attacks occurred in a largely Muslim region known for religious tolerance but tension has been increasing betweenthe moderate Islamic clerical leadership and those who practice the puritanical version of Islam known as Salafism, the Journal reported.
Faizov was elected Tatarstan's chief mufti last year and has warned that the Salafists, who have thousands of followers in the region, were a growing and potentially violent threat, the Journal said.
The official Web site for Tatarstan called Yakupov the "strategist behind Faizov's policy of rooting out religious extremism."
Russia's Investigative Committee called the attacks, which occurred 20 minutes apart, acts of terrorism.