Yet another suspect in the attack was detained Tuesday. Police said Monday they detained 20 suspects.
Prosecutors have issued eight arrest warrants, and another five are pending.
Police said earlier they are investigating the incident as "intentional grievous bodily harm resulting in death."
On Saturday, attackers wearing face masks and wielding baseball bats and metal rods raided a camp of environmental activists in the city of Angarsk, near Lake Baikal, who were there protesting nuclear waste processing at a Siberian chemical plant.
Police said the suspects were young people under 22, primarily students and unemployed. There were 21 environmentalists in the camp.
Environmentalists have previously protested against the state-owned Angarsk Electrolysis Chemical Plant, which processes uranium for the nuclear industry.