That document was allegedly arranged by ex-Trump campaign chairman Paul Manafort, who worked for Viktor Yanukovych, the former pro-Russian president of Ukraine.
The firm, which is co-operating with the investigation and which fired him a year ago, worked with former Trump campaign chief Paul Manafort and his ex-colleague Mr Gates at a time when they were both political consultants in Ukraine.
Van der Zwaan had faced zero to six months in prison under federal sentencing guidelines, and his attorneys had pushed for him to pay a fine and leave the country. John Dowd, Trump's former attorney, called for Mueller's probe to be shut down.
Several other Trump campaign associates have also pleaded guilty to various charges stemming from the special counsel investigation, including Gates, former Trump campaign adviser George PapadopoulosGeorge Demetrios PapadopoulosPapadopoulos encouraged by Trump campaign staffer to make contact with Russians: report House Intel votes to release report in Russia probe Did McCabe commit a crime? This, according to the indictment, was a lie - van der Zwaan was actually secretly communicating with Gates and Person A about the Skadden report.
Van der Zwaan was the fourth guilty plea in the Mueller investigation after Trump campaign aide George Papadopoulos and former national security adviser Michael Flynn pleaded guilty to lying to investigators previous year and agreed to cooperate, and an online supplier of fake usernames pleaded guilty to identity fraud in February.
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Van der Zwaan is married to the daughter of German Khan, a billionaire who owns Russia's largest financial investment company, the Alfa Group. So they retained a team from Skadden Arps, which included van der Zwaan, to put together a "report" that conveniently concluded that there was no political motive for putting her in jail. "Because investigation of those matters was authorized, so was prosecution".
But in their new filing, prosecutors revealed that Rosenstein - who appointed Mueller - wrote a memo last August that outlined the scope of Mueller's appointment. After speaking with the special counsel's office in a voluntary interview in November and confronted with the email he had hid, van der Zwaan was subpoenaed by Mueller to appear before a grand jury.
Mr Gates and Mr Manafort have been hit with a number of charges, including conspiracy to launder money and acting as "unregistered agents" of Mr Yanukovych and his party. In court papers filed last week, it was disclosed that Gates was talking with a former Russian intelligence officer just before the 2016 election.
President Trump has said that the charges against his former employees have no bearing on his campaign's work and repeatedly claims that "no collusion" occurred. He has not been indicted on any charges related to election interference.