Special counsel Robert Mueller gained another guilty plea Tuesday as a Russian oligarch's son-in-law admitted lying to the FBI – after another figure connected to former Trump campaign chair Paul Manafort faced charges.

Alex van der Zwaan pleaded guilty at federal court in Washington D.C. in a plea deal with the special counsel.

The 33-year-old becomes the fourth to be convicted as a result of the Mueller investigation, coming after President Donald Trump's disgraced national security adviser Mike Flynn and campaign adviser George Papadopolous both cut deals with the special counsel. Richard Pinedo, a California man, pleaded guilty on Feb. 12 to identity fraud.

Also charged in the probe is former Manafort deputy Rick Gates, who features in Tuesday's indictment as party to a phone call that Van Der Zwaan secretly recorded.

Disgraced: Attorney Alex Van Der Zwaan is expected to plead guilty to lying to Robert Mueller's investigators. he arrived at the Washington D.C. federal courthouse ahead of a hearing where he pleaded guilty to lying to the FBI

Van der Zwaan, 33, is a Dutch citizen and London-based lawyer whose father-in-law, German Khan, is named in the notorious 'golden showers' dossier on Trump which was drawn up by British spy Christopher Steele.

Van der Zwaan's law firm assisted in efforts on behalf of Ukrainian former president Viktor Yanukovych - the Putin ally forced from office in the country's 'orange revolution'.

Plea deal: Alex van der Zwaan, a London-based, Dutch-born attorney for a high-profile New York firm, is pleading guilty to lying to the FBI and destroying evidence by deleting emails

Alex Van Der Zwaan wedding to Eva Khan. Circled is her father, German Khan, a Russian billionaire who is suing over being named in the 'golden showers' anti-Trump dossier

Robert Mueller is expected to get a guilty plea Tuesday from Alex Van Der Zwaan, who will plead guilty to making 'false, fictitious, and fraudulent statements' to investigators about his dealings with Rick Gates (right), Paul Manafort's deputy

Van der Zwaan did work as an associate for the law firm Skadden, Arps, Meagher, & Flom LLP on behalf of the project.

Paul Manafort, a former campaign chair for the Trump campaign, has been already been charged with conspiracy and money laundering.

But when he got asked about his activities during a November interview with authorities, Van Der Zwaan 'made materially false statements,' according to a statement of offense. His own lawyer was present during the interview, according to the government.

Prosecutors wrote that at all relevant times, Manafort was under investigation for foreign principles without registering registering as required by lobbying laws.

Van Der Zwaan admitted that he wasn't truthful about his dealings with Rick Gates, Manafort's deputy, who is also charged and has been in negotiations about cooperating with prosecutors.

He was ensnared by his own notes, emails, and recordings – which he failed to turn over to his law firm when prosecutors asked for them.

Charging documents show that he surreptitiously recorded Gates but lied to the FBI about it.

According to a statement of offense, Gates called Van Der Zwaan in September 2016 and told him to contact 'Person A' – who prosecutor Andrew Weissman identified in court as a Ukraine-based Manafort associate.

'After the call, Gates sent Van Der Zwaan documents including a preliminary criminal complaint in Ukraine via an electronic application called Viber,' according to the statement. Van Der Zwaan then called Person A and discussed in Russian that formal criminal charges might be brought against a former Ukrainian Minister of Justice, Law Firm A, and Manafort.

Van Der Zwaan recorded the call, according to the statement, then called the senior partner at his firm and partially recorded that call, before calling Gates and recording that call as well, taking notes each time.

But when asked for notes or emails by prosecutors, Van Der Zwaan 'did not produce' them to the law firm and 'deleted and otherwise did not produce emails he possessed that he understood had been requested by either the Special Counsel's Office or Law Firm A, or both.'

Among the emails he didn't hand over was one, dated Sept. 12, 2016, written in Russian, where the person had asked Van Der Zwann to call using an encrypted app.

Although the documents do not identify 'Person A,' Mother Jones reported the person could be Konstantin Kilimnik, who was Manafort's Ukraine-based business partner.

Manafort reached out to Kilimnik in April, soon after he was brought aboard the Trump campaign. '"How do we use to get whole?' Manafort wrote. His attorney, Jason Maloni, told the Washington Post it was a reference to collecting on debts.

Guilty pair: Mike Flynn, Trump's disgraced national security adviser (left), and ex-Trump campaign aide George Papadopolous were the first two to cut deals with Robert Mueller

He lied when he said his last communication with Gates was in August 2016, according to the government, when in fact in September 2016 'he spoke with both [Manafort deputy Rick] Gates and Person A' about a report and 'surreptitiously recorded the calls.'

At the time, Gates was deputy campaign manager of Donald Trump's campaign.

Van Der Zwaan had said that his last communication consisted of an innocuous text message, according to investigators.

Van Der Zwaan is the son-in-law of Russian oligarch German Khan. He is estimated by Forbes to be worth $10 billion.

Khan is one of three oligarchs to appear in the infamous Steele Dossier who have sued a political intelligence firm over its release. They also sued BuzzFeed for publishing the dossier.

Khan was joined by two other oligarchs, Mikhail Fridman, Petr Aven, in suing Fusion GPS, the firm that contracted with ex British intelligence officer Christopher Steele, who compiled the dossier.

They are majority owners of the Russian bank Alfa Bank, which the dossier reports aided the Russian election meddling effort – though notably misspelling the bank's name as 'Alpha Bank.'

The dossier contained sensational unverified claims about Donald Trump's supposed conduct in a Moscow Hotel room during a visit for the Miss Universe Pageant, but it also focused on Trump's business dealings.

According to the dossier: 'Top level Russian official confirms current closeness Of Alpha Group-PUTIN relationship. Significant favours continue to be done in both directions,' according to the compilation of Steele memos.

Van Der Zwaan was charged Feb. 16 in federal court in the U.S. District Court for the District of Columbia. He appeared Tuesday at the courthouse to submit his plea.

TO HAVE AND TO HOLD: Alex Van Der Zwaan married Eva Khan, the daughter of Russian oligarch, German Khan, outside London

The oligarch walked his daughter down the aisle

Alex Van Der Zwaan's home in Bayswater in West London. He lives in one of London's most exclusive areas and his father-in-law is estimated to be worth at least $10 billion

He also 'deleted and otherwise did not produce' emails sought by the special counsel and a law firm – thereby withholding information from the probe, which has already led to multiple guilty pleas, and last week unveiled an indictment of 13 Russians working for a troll farm attempting to influence the 2016 presidential election.

Van Der Zwaan is the son-in-law of Russian oligarch German Khan, the Kyiv Post reported.

Van Der Zwaan married Eva Khan, German Khan's daughter, at Luton Hoo Hotel, a posh estate outside of London. The festivities lasted three days.

Skadden Arps said they terminated Van Der Zwaan in 2017, that he was cooperating with Mueller, and that he had worked for them for a decade.

The Los Angeles Times reported Sunday that Gates will himself plead guilty to fraud charges and will agree to testify against his longtime partner, Manafort.

The information provided by the government says Van Der Zwaan's false statements were made in connection to his work as an attorney working for a law firm whose client was the Ukraine Ministry of Justice. The firm was helping prepare a report on the trial of Yulia Tymoshenko.

Tymoshenko is a former Ukrainian prime minister who was tried and jailed for fraud in proceedings European governments labeled politically motivated.

He was part of a team dispatched by his firm, working along with Manafort, to bolster Yanukovych's contention that the jailing of his political rival was not politically motivated.

The Justice Department asked the firm for documents related to the report, the New York Times reported in September.

Ukrainian prosecutors sought to question Greg Craig, who was Skadden Arps' lead attorney on the case, and who served as President Obama's White House counsel. They also wanted to question Clifford Sloan, another Obama administration official.