US congressional investigators have launched an inquiry into possible links between British-based companies owned by Russian oligarchs and their relationship with Russia’s intelligence agencies.

The move follows increasing concerns by U.S. congressmen over the extent of Russian meddling in both the U.S. and Europe, with a number of oligarchs with close ties to Russian President Vladimir Putin forming the main focus of the inquiry.

Among the UK-based companies that have aroused the interest of congressional investigators is EN+, the energy company owned by Oleg Deripaska and chaired by Tory peer Lord Barker of Battle.

Mr Deripaska, a close ally of Mr Putin, is already under investigation from congressional committees over allegations, which the oligarch denies, that he was involved in efforts to interfere with the 2016 presidential elections.

Recent documents released by the FBI revealed Mr Deripaska loaned $10 million to Paul Manafort, Donald Trump’s campaign manager, who has been charged with fraud and money-laundering. Mr Deripaska is one of several Russian oligarchs who were hit with U.S. sanctions in April.

Now US investigators say they are interested in apparent links between British-based companies owned by Russian oligarchs and Russian intelligence agencies.

Their interest in EN+ comes after FBI officers identified Evgeny Fokin, who is the company’s Director of International Cooperation, as formerly being the SVR’s declared liaison officer with U.S. intelligence agencies in Washington DC in the mid-1990s.