Sam Nunberg, a former political aide to Donald Trump, will be interviewed on Thursday as part of a US special counsel's investigation of Russian meddling in the 2016 election, two people with knowledge of the matter said.
George Papadopoulos, who was on the Trump campaign's foreign policy advisory panel, admitted in October 2017 to making false statements to Federal Bureau of Investigation agents about contacts he had with the Russian government in 2016.
Manafort and Gates both pleaded not guilty after their indictment last fall.
CNN previously reported that Gates has been in negotiations with Mueller's office for more than a month, and that Gates had spoken to prosecutors about his case since his indictment. Both men have pleaded not guilty.
Mueller alleges that Manafort and Gates "owned or controlled" 33 different entities that they used to further their scheme. This particular report was authored by the Skadden Arps. Last year, van der Zwaan married the daughter of Ukrainian-Russian billionaire German Khan, according to the Russian editions of Forbes and Tatler magazines.
References to those loans have popped up in recent court filings related to the negotiations over Manafort's bail proposal, which involves properties he had put up as collateral for the FSB loans.
The charging document said that while answering Federal Bureau of Investigation agents' questions about his work on the report, van der Zwaan lied that he last communicated with Gates in mid-August and another unnamed person, only identified as "Person A", in 2014.
Mueller's office declined to comment.
The description given by prosecutors for Person A appears to match Russian-Ukrainian political operative Konstantin Kilimnik, a long-time employee of Manafort's political consulting operations. American media claim that the man - in-law of Russian businessman German Khan.
Former national security adviser Michael Flynn and former campaign foreign policy aide George Papadopoulos have both pleaded guilty to lying to the Federal Bureau of Investigation about their contacts with Russians.
Mueller explained in a status report that some of the new charges could not be brought in Washington without the approval of the defendants.
In a statement on Tuesday, a spokeswoman for the firm said it had terminated van der Zwaan's employment in 2017 and "has been cooperating with authorities in connection with this matter".
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