A lawyer who previously worked with Paul Manafort, Trump’s former campaign manager, was charged with – and has plead guilty to – lying to the FBI, marking another major development the investigation into possible collusion between the Trump election campaign and the Kremlin.

Alex van der Zwaan, who married the daughter of a Russian-Ukrainian oligarch last year, admitted making false statements in connection to work he did in Ukraine, as part of a plea agreement with the special prosecutor on the Russia investigation, Robert Mueller.

It is unclear how – if at all – the case against Van der Zwaan is connected to the broader question of whether or not the Trump campaign conspired with the Kremlin. But the indictment shows that one focus of the Mueller investigation is Manafort’s business dealings with pro-Russia figures in Ukraine prior to becoming Trump’s campaign manager in 2016.

According to court documents filed in Washington and made public on Tuesday, Van der Zwaan failed to disclose a series of contacts in September 2016 with Rick Yates, a business associate of Manafort, and person identified only as Person A, with whom Van der Zwaan spoke in Russia and who was described as a “longtime business associate of Manafort and Gates in Ukraine”.

That description appeared to match Konstantin Kilimnik, a former Russian army linguist whom federal prosecutors have “assessed to have ties” with Russian intelligence. Kilimnik ran Manafort’s office in Kiev after Manafort left Ukraine in 2014, and continued his work for pro-Russia interests in Ukraine.

The court documents allege that the undisclosed contacts in 2016 concerned work performed four years earlier by the US law firm Van der Zwaan worked for in London – Skadden, Arps, Slate, Meagher & Flom – on behalf of the former pro-Russia Ukrainian government of Viktor Yanukovych. Manafort hired Skadden to defend Yanukovych from western condemnation after the 2011 detention of an opposition leader.

According to the court documents, Gates told Van der Zwaan to contact Person A in September 2016 to discuss impending criminal charges in Ukraine against Manafort, a former Ukrainian minister of justice and “law firm A”, an apparent reference to Skadden.

In particular, Van der Zwaan failed to tell prosecutors or his…