A Justice Department lawyer said Thursday that former Trump campaign chairman Paul Manafort Paul John ManafortOur Constitution is under attack by Attorney General William Barr Bannon trial date set in alleged border wall scam Conspicuous by their absence from the Republican Convention MORE is being investigated as a possible “back channel” to Russia.

At a hearing, federal prosecutors defended special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE’s investigation into whether the Trump campaign colluded with Russia's meddling in the 2016 presidential election, arguing that Mueller did not exceed his authority by indicting Manafort on charges related to his pre-campaign work.

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Michael Dreeben, an attorney for the Justice Department, told U.S. District Judge Amy Berman Jackson that part of Mueller’s probe into Manafort is investigating whether the former campaign chair provided “back channels” to Russia, according to Bloomberg.

“He had long-standing ties to Russia-backed politicians,” Dreeben said. “Did they provide back channels to Russia? Investigators will naturally look at those things.”

Prosecutors said earlier this month in a court filing that Mueller’s investigation, by nature of investigating connections 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.”

Manafort is charged on several counts, including money laundering, tax fraud and other financial crimes, related to his and his associate Richard Gates’s lobbying work for a pro-Russia political party in Ukraine. Gates has pleaded guilty to two counts and is cooperating with the investigation.

The Justice Department reportedly authorized Mueller to investigate allegations of collusion between Manafort and the Russian government.

Earlier this month, Manafort filed a civil lawsuit attempting to limit the scope of Mueller’s probe, arguing that the special counsel overstepped his boundaries by investigating allegations that arose before his time on the Trump campaign.

Mueller has indicted 13 Russian nationals and three Russian entities on charges related to alleged meddling in the presidential election.