Five suspects have been detained by Russian authorities over attacks that wounded the top Islamic official in the mostly Muslim Tatarstan region, killed his deputy and raised fears of the spread of militancy to Russia's heartland.
The federal Investigative Committee on Friday suggested that the attacks - in an area previously held up as a model of religious tolerance - were provoked by disputes over faith and money.
Tatarstan's mufti, Ildis Faizov, was rushed to hospital after three powerful blasts hit his car in Tatarstan's capital, Kazan, on Thursday.
A little earlier, deputy mufti Valiulla Yakupov was shot dead outside his home.
President Vladimir Putin and top security officials called at the weekly meeting of his Security Council for tough measures against extremism after the attacks.
Participants underscored the need to "step up opposition to all forms of extremism, including religious," state-run news agency RIA quoted Putin's spokesman Dmitry Peskov as saying.
Fayzov's car was blown up as he left a radio station in the city of Kazan, about 735km east of Moscow, the federal Investigative Committee said in a statement.
There were no immediate claims of responsibility for the attacks.
The attacks evoked the deadly violence that plagues mainly Muslim regions of the North Caucasus, where Russian forces have fought rebels in two devastating wars since the 1991 Soviet collapse and fighters want to carve out an Islamic state.
Oil-producing Tatarstan, which has a higher degree of autonomy from Moscow than most regions and has a majority ethnic Tatar population, is relatively peaceful. It has a large Muslim population.
Its historic capital Kazan, on the Volga River, has been chosen as the host city for the World University Games next year.
"Investigators believe the main motive was the professional activity of the victims, including their ideological differences with opponents," investigators said in a statement.
Faizov had taken "a tough position toward organisations that preach radical forms of Islam", they said.
"In addition, he took control of the movement of financial resources of the organisation Ideal-Hadzh, which sent Muslims to Mecca, and on this basis a conflict occurred between the mufti and the leader of this organisation, which threatened him."
It said the chairman of Ideal-Hadzh, Rustem Gataullin, 57, was among those detained, along with the leader of a Muslim place of worship, Murat Galleyev, an Uzbek citizen and two other residents of Tatarstan.
Putin, who has emphasised the need for religious tolerance and unity in a mainly Orthodox Christian country with a large Muslim minority, promised on Thursday that the culprits would be found and punished.
"It is a serious signal," Putin said of the attacks, carried out hours before the Muslim holy month of Ramadan began at sundown on Thursday.