The editor of a prominent Russian opposition newspaper has accused the country's top investigator of threatening to kill one of the newspaper's journalists.

Dmitry Muratov, editor of Novaya Gazeta, claimed in an open letter that Investigative Committee chief Alexander Bastrykin made the threat against reporter Sergei Sokolov.

Muratov wrote that Bastrykin's threat concerned a June 4 article in which Sokolov accused Russia's top investigator of failing to properly punish those responsible for a 2010 killing of 12 people, including children, by a gang in southern Russia.

Muratov accused Bastrykin's security guards of putting Sokolov into a car on the same day the article appeared and driving him to a forest where, according to Muratov, Bastrykin made the threat.

The editor said Sokolov left the country following the threat.

The Investigative Committee declined to comment on the letter.

Witnesses say five journalists were detained on Wednesday as they tried to picket the Investigative Committee building.

Novaya Gazeta is known for its investigative reporting into government corruption and its opposition to President Vladimir Putin. The paper has lost several journalists in unresolved murders in the past decade.