MOSCOW (Reuters) - A group of Russian journalists who investigated the activities of a secretive group of Russian mercenaries in Africa and the Middle East have been subject to a campaign of physical threats and harassment, their editor-in-chief said.

“Proekt,” an independent Moscow-based online news outlet which specializes in investigations, began to publish a series of articles in March looking into the role of a shadowy group of mercenaries known as Wagner.

Around the same time editor-in-chief Roman Badanin said his journalists began to receive emailed threats of physical retribution for their work.

Unknown people tried to break into his staff’s personal accounts on Facebook, the Telegram messenger service and Google mail, he said, and one of his journalists was followed in the street by an unknown man who filmed her with a video camera.

“This is all simply an attempt to make us nervous, to distract from our journalistic work, to make it clear that we’re under surveillance and that they’re watching us,” Badanin told Reuters in an interview.

Badanin said he could not prove who was behind the harassment campaign, which he said peaked last month when Proekt ran an investigation into Wagner’s apparent activities in Libya.

Proekt did not complain to the police over the incidents, he said, saying he had instead decided to speak out publicly to draw attention to the threats.

People with ties to the Wagner group have previously told Reuters that it has carried out clandestine combat missions on the Kremlin’s behalf in Ukraine and Syria. Russian authorities deny Wagner contractors carry out their orders.

The Wagner group was thrust into the spotlight last year when three Russian journalists were killed in Central African Republic while investigating there.

Russian private military contractors use a defense ministry base in southern Russia containing barracks that were built by a company linked to businessman Yevgeny Prigozhin, Reuters reported in April.

The United States imposed sanctions on Prigozhin after it accused him of trying meddle in its 2016 presidential election and the 2018 U.S. congressional elections.

Prigozhin has denied any links to Wagner and has declined to comment on the U.S. allegations, calling them a “private matter” for the U.S. Treasury.

When asked if it was behind Proekt’s troubles, Concord Management and Consulting, Prigozhin’s main business, said by email:

“If journalists are being persecuted or threatened they should phone 02,” a reference to the police emergency number in Russia.

Russia is regularly criticized by media freedom watchdogs who say attacks on journalists often go unpunished. The New York-based Committee to Protect Journalists says 28 journalists have been killed in Russia since 2000.