Prosecutors working for the special counsel Robert Mueller told a federal judge they have long suspected former Trump campaign chairman Paul Manafort of serving as a "back channel" between the campaign and Russia during the 2016 election.

The revelation came during a hearing this week over whether Mueller overstepped his authority when he charged Manafort with crimes related to his Ukraine lobbying work and not Russian collusion.

Manafort's lawyers also argue that deputy attorney general Rod Rosenstein gave Mueller an overly broad mandate.

Prosecutors told a federal judge this week that they suspected Paul Manafort, the former chairman of President Donald Trump's campaign, of serving as a "back channel" between the campaign and Russia during the 2016 US election.

The special counsel Robert Mueller's office has charged Manafort with 23 counts related to money laundering, failing to register as a foreign agent, tax evasion, bank fraud, and conspiracy against the US. Most of the charges so far center around Manafort's lobbying work for the Ukrainian government, the pro-Russian Party of Regions, and former Ukrainian president Viktor Yanukovych.

Manafort's attorneys have mounted an aggressive defense against Mueller's office since then, arguing that deputy attorney general Rod Rosenstein gave Mueller an overly broad mandate, and that Mueller overstepped his authority when he charged Manafort for crimes unrelated to Russian collusion.

Kevin Downing, Manafort's main defense lawyer, argued to US District Judge Amy Berman Jackson in Washington, DC that Rosenstein's order appointing Mueller, which authorizes him to investigate Russia's interference in the election and "any matters that arose or may arise directly from the investigation," does not cover Manafort's consulting work in Ukraine.

Michael Dreeben, a seasoned prosecutor working on Mueller's team, pushed back and said they were justified in probing Manafort because of his status as Trump's campaign chairman and his "long-standing ties to Russia-backed politicians."

"Did they provide back channels to Russia?" Dreeben said. "Investigators will naturally look at those things."

When Trump first hired Manafort in March 2016, Manafort was known for having worked as a top consultant to Yanukovych, and is widely credited with helping him win the election in 2010.

Yanukovych, a strongman and a prominent figure in the Party of Regions, was ousted from the presidency in 2014 amid widespread protests against his Russia-friendly positions and his decision to back out of a deal that would have promoted closer ties between Ukraine and the West, and distanced it from Russia.

A long history of ties to Russian wealth

Paul Manafort, President Donald Trump's former campaign chairman, departs Federal District Court, Thursday, Nov. 2, 2017, in Washington. AP Photo/Andrew Harnik

Manafort also had his share of financial entanglements with entities related to Russia when he joined the Trump campaign.

In July, The New York Times reported financial records Manafort filed in Cyprus that showed he was $17 million in debt to pro-Russian interests when he joined the campaign. Shell companies connected to Manafort during his time working for the Party of Regions bought the debt, according to The Times.

Manafort is also associated with Oleg Deripaska, a wealthy Russian oligarch. Deripaska's representatives claimed, in legal complaints filed in the Cayman Islands in 2014, that Manafort had disappeared after Deripaska gave him $19 million to invest in a failed Ukrainian TV venture.

Deripaska is a longtime ally of the Kremlin and Russian President Vladimir Putin. Manafort's relationship with Deripaska, meanwhile, stretches back years and relates to the two men's work pushing pro-Russian interests on the world stage.

Deripaska and Manafort worked together in 2006, when Deripaska signed a $10 million annual contract with Manafort for a lobbying project in the US that Manafort said would "greatly benefit the Putin Government."

But things haven't always been rosy between them. Deripaska's representatives claimed, in legal complaints filed in the Cayman Islands in 2014, that Manafort had disappeared after Deripaska gave him and his longtime associate Rick Gates $19 million to invest in a failed Ukrainian TV venture in 2007.

Last year, The Washington Post reported that Manafort emailed the former Russian military intelligence operative Konstantin Kilimnik, beginning in April 2016, offering to give Deripaska "private briefings" about the Trump campaign. Former intelligence officials told Business Insider that the offer was likely part of Manafort's effort to resolve his financial dispute with Deripaska.

Manafort and Kilimnik met in May and August during the 2016 campaign.

Three days before his August meeting with Manafort, Kilimnik wrote in an email to the Trump campaign chairman that he had "met today with the guy who gave you your biggest black caviar jar several years ago," a reference to Deripaska's previous loans to Manafort. "We spent about 5 hours talking about his story, and I have several important messages from him to you," he wrote.

"I need about two hours because it is a long caviar story to tell," he added.

Manafort said he and Kilimnik discussed the Trump campaign and the recent hack of the Democratic National Committee during the August 2, 2016 meeting. Kilimnik, meanwhile, said they did not discuss the campaign, but talked about "current news" and "unpaid bills."

Within hours of the meeting, a private jet linked to Deripaska arrived in Newark, New Jersey, which is close to where Manafort and Kilimnik met. It left within 24 hours.

Manafort was also one of three Trump campaign officials to meet with two Russian lobbyists offering kompromat on Democratic nominee Hillary Clinton in June 2016.

Mueller and Manafort duke it out

Robert Mueller. Alex Wong/Getty Images

Mueller's office said in an April 2 court filing that any investigation into ties between the Trump campaign and Russia "would naturally cover ties that a former Trump campaign manager had to Russian-associated political operatives, Russian-backed politicians, and Russian oligarchs."

"It would also naturally look into any interactions they may have had before and during the campaign to plumb motives and opportunities to coordinate and to expose possible channels for surreptitious communications," it added. "And prosecutors would naturally follow the money trail from Manafort's Ukrainian consulting activities. Because investigation of those matters was authorized, so was prosecution."

Meanwhile, Downing, Manafort's attorney, called into question the timing of a memo Rosenstein sent to Mueller in August 2017. The memo authorizes Mueller to investigate specific allegations related to Manafort, including his Ukraine lobbying work and possible collusion with Russia.

Rosenstein appointed Mueller special counsel last May, and Downing suggested Rosenstein sent the memo almost three months later because he had failed to properly outline the scope of Mueller's investigative focus at the time of his appointment, as mandated by Justice Department regulations.

Rosenstein sent the memo in August, Downing said, because "he realizes he got something wrong" when appointing Mueller in May.

Dreeben countered Downing's argument and said the August memo confirmed what prosecutors suspected about Manafort's activities when Mueller was appointed, adding that Mueller is required to regularly report on the status of the investigation and its findings to Rosenstein.

"It's not a blank check," Dreeben said. "It's not a carte blanche."