Two of those detainees have been charged with premeditated infliction of bodily harm leading to death, German Strublin, the Irkutsk regional police spokesman, told The Associated Press. Authorities have not decided whether to charge the others, he said.
Witnesses said the attackers were dressed in dark clothes, wielded metal pipes and shouted nationalist slogans Saturday as they rampaged through a camp in a forest near the city of Angarsk, about 4,200 kilometers (2,600 miles) east of Moscow.
Strublin said that police believe the attack may have been triggered by a personal argument between one of the environmentalists and one of the attackers. They do not believe it was linked to the protest, he said.
The 21 environmentalists were protesting over a chemical plant outside Angarsk, which processes uranium for the nuclear power industry. Activists charge that Russia plans to become a center for processing and storing spent fuel from abroad, and that the Angarsk plant could be part of the lucrative business.
A wave of hate crimes has swept Russia in recent years. Rights activists say nationalist groups are emboldened by the reluctance of the authorities to take tough measures against those responsible.
Police said earlier that the attackers were young people, including the unemployed and students.