The federal Investigative Committee suggested the attacks – in an area in Russia previously held up as a model of religious tolerance – were provoked by disputes over faith and money.
The Tatarstan region's mufti, Ildis Faizov, was hospitalised 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.
The attacks evoked the deadly violence that plagues mainly Muslim regions of the North Caucasus, where federal forces have fought rebels in two devastating wars since the 1991 Soviet collapse and militants want to carve out an Islamic state.
Militants in the Caucasus sometimes target mainstream Muslim leaders backed by the authorities. The attacks in Tatarstan – on the Volga River east of Moscow and far from the Caucasus – suggested similar tensions may be deepening there.
"Investigators believe the main motive was the professional activity of the victims, including their ideological differences with opponents," the Investigative Committee said in a statement.
Faizov had taken "a tough position toward organisations that preach radical forms of Islam" since his election in April 2011, it 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 chairperson of Ideal-Hadzh, Rustem Gataullin (57) was among the detained suspects, along with the leader of a Muslim place of worship, Murat Galleyev and two other residents of Tatarstan. The suspects would be held as the investigation continued, the Investigative Committee said.
President Vladimir Putin, who has emphasised the need for religious tolerance and unity in a mainly Orthodox Christian country with a large Muslim majority, issued a statement on Thursday promising the culprits would found and punished.
But some experts were sceptical that militant ideology motivated the attacks, in a region were it is rare to see women wearing headscarfs or Muslim dress.
"Any terrorist attack is easy to attribute to extremist, but I think this is a prosaic problem," sociologist Enver Kisiriev, an expert on the North Caucasus at the Russian Academy of Sciences, told Reuters.
"The true concern of political and any other elite in Russia today – that is money." – Reuters