Deputy Attorney General Rod J. Rosenstein outlined the scope of the special counsel investigation in a confidential memo in August 2017. (Win Mcnamee/Getty Images)

Special counsel Robert S. Mueller III was authorized by a top Justice Department official to investigate whether Paul Manafort, the onetime chairman of Donald Trump’s presidential campaign, illegally coordinated with Russia to interfere in the 2016 election, new court filings show.

Manafort, who was indicted last year on felony charges related to his work in Ukraine before joining Trump’s campaign, has not been charged with any crimes connected to the presidential race. But a partly redacted memo included in court filings late Monday night revealed that Deputy Attorney General Rod J. Rosenstein authorized Mueller to pursue allegations that Manafort colluded with Russia in 2016.

The new filings show that Rosenstein specifically approved lines of investigation for the special counsel in an August 2017 memo. A version of the memo filed in court showed that Rosenstein signed off on an investigation of whether Manafort “committed a crime or crimes by colluding with Russian government officials” and of Manafort’s work as an international political consultant in Ukraine before joining Trump’s campaign.

Additional sections of the 2 1 / 2 -page memo were blacked out by prosecutors, indicating that Rosenstein authorized other lines of investigation that remain a secret.

Mueller was appointed in May to examine whether Trump associates participated in a Russian effort to swing the election. But his team has not previously identified Manafort, who was intimately involved in the campaign’s operation, as a specific target of that inquiry.

Manafort, 69, served as Trump’s campaign chief for five months. He resigned in August 2016 amid news reports about his activities in Ukraine.

He has been charged with felony counts related to financial offenses — including money laundering, and fraud and tax evasion, as well as with not registering as a foreign lobbyist — in connection with his decade of work on behalf of a Russia-friendly Ukrainian political party before joining Trump’s campaign. He has pleaded not guilty and is seeking to have the case dismissed.

Manafort’s longtime deputy, Rick Gates, 45, was charged jointly with Manafort but pleaded guilty to reduced charges last month and is cooperating with Mueller’s inquiry.

Mueller’s team revealed the existence of the Aug. 2, 2017, memo in filings that seek to counter arguments by Manafort’s attorneys that the special counsel investigation has exceeded its legal authority.

Manafort’s legal team has argued that Rosenstein inappropriately gave the special counsel’s office a “blank check” when he authorized Mueller in May to investigate any matter that “arose or may arise” from an examination of the Trump team’s ties to Russia.

In particular, they argued that prosecutors’ knowledge of Manafort’s work in Ukraine did not arise directly from Mueller’s inquiry but instead grew out of separate Justice Department inquiries dating to 2014. Manafort’s lawyers have alleged in court documents that Mueller’s investigation has become an “unbounded exercise of prosecutorial authority [that] is wholly incompatible with our constitutional tradition.”

In a detailed, 53-page defense, Mueller’s team countered late Monday that an investigation of possible links between the Trump campaign and Russia would “naturally cover ties” between the campaign’s chairman and “Russian associated political operatives, Russian-backed politicians and Russian oligarchs.”

In Ukraine, Manafort worked on behalf of then-President Viktor Yanukovych, who was backed by Russia, and had personal business dealings with Russian aluminum magnate Oleg Deripaska.

Prosecutors argued that their investigation would only “naturally follow the money trail from Manafort’s Ukrainian consulting activities,” work that resulted in the criminal charges.

What’s more, they said that although Rosenstein’s public order appointing Mueller did not name individual subjects, Rosenstein specifically authorized the Manafort probe in the follow-up August memo. They also noted that Rosenstein has said publicly that he is supervising the investigation closely and that he does not think Mueller has exceeded his authority.

Rosenstein, who has become a particular target of Trump’s ire, has supervised the Russia inquiry since Attorney General Jeff Sessions recused himself because he was an adviser to Trump’s campaign.

Prosecutors did not explain the timing of the August memo. It came not long after the FBI raided Manafort’s condominium in Alexandria, Va., and served him a search warrant in late July. The memo was written a few days after the July 27 arrest of George Papadopoulos, who had served as a foreign policy adviser to the campaign.

Papadopoulos has pleaded guilty to lying to the FBI about his Russia-related contacts during the campaign, including his communication with a London-based professor who told Papadopoulos that the Russians held thousands of emails that would be damaging to Democratic presidential candidate Hillary Clinton.

Trump has repeatedly insisted that Mueller’s investigation has found “no collusion” on the part of his campaign.

However, Mueller’s team has offered hints that investigators continue to scrutinize possible coordination between Trump associates and Russia.

In a court filing last week, prosecutors indicated that the FBI has assessed that a longtime Manafort business associate who was in contact with both Manafort and Gates during the campaign had ties to Russian intelligence. They also said that a London-based lawyer who worked with Gates has told prosecutors that Gates knew the man had been a Russian intelligence officer.

A hearing on the motion to dismiss charges against Manafort is set for April 19. In the meantime, he faces two pending trials related to his work in Ukraine: one in Virginia, scheduled to begin in July, and the other in Washington, set to take place in September.