Russian police have arrested two people suspected of killing a top human rights lawyer and a journalist last January.
Stanislav Markelov, who had worked on numerous human rights cases, was shot dead in broad daylight in Moscow with reporter Anastasia Baburova.
Those detained are both Moscow residents, a man and a young woman, Russian investigators said.
There has been a series of killings of human rights workers and journalists in recent years in Russia.
The suspects were named as Nikita Tikhonov and Yevgenia Khasis.
Law enforcement sources linked them to an outlawed far-right group, Russian National Unity.
But the group itself denied any involvement in the murders and said those arrested had never been members.
Stanislav Markelov and Anastasia Baburova, who worked on the opposition Novaya Gazeta newspaper, were shot near the Kremlin by a masked gunman on 19 January.
Novaya Gazeta specialises in human rights issues and used to employ Anna Politkovskaya, the investigative reporter who was herself murdered in 2006.
Mr Markelov had worked on many high-profile human rights cases and had represented Anna Politkovskaya.
He was killed shortly after holding a news conference on another prominent case - that of Elza Kungayeva, a Chechen woman killed by a Russian army officer in 2000.
Mr Markelov, who represented her family, had promised to challenge a court ruling giving the officer early release from prison.
At the time of his death, Mr Markelov was also involved in the case of a newspaper editor from Khimki near Moscow who was beaten up by unknown attackers.