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 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.

Police on Wednesday arrested five journalists who picketed the Investigative Committee headquarters in Moscow to protest the alleged threat against Sokolov.

Matthew Schaaf, a Eurasia analyst with a U.S.-based rights group, Freedom House, said Muratov's accusations are a “cause for concern” if they prove true.

“If those accusations do turn out to be true, it's exemplary of an environment in Russia where government officials, where different government organizations and government bodies do seek to influence the press and pressure the press in any number of different ways. So if it turns out to be that this case, this confrontation did in fact occur in the way that it's been recounted, then it is definitely a cause for concern.”

Schaaf said that while Russian journalists are often threatened, with some physically attacked or even murdered, Freedom House rarely hears of Russian officials making direct threats against journalists.

“Now Russia is unfortunately very well known for physical attacks and even murders against journalists. However, many of the cases are unsolved and it's unclear exactly who was involved, who ordered many of these murders, so it fits very well in with an environment that's very hostile to journalists and that is even threatening to the lives of journalists. But we haven't seen very often the direct threats toward journalists as are recounted in this letter about the meeting with the director of the Investigative Committee.”

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.