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The Manhattan district attorney's office is moving forward in preparing a case against Paul Manafort in connection with state tax and bank fraud-related charges, a source familiar with the matter told NBC News.

The developing state case against Manafort, President Donald Trump's former campaign chairman, was first reported by Bloomberg and The New York Times.

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A source confirmed to NBC News that a state grand jury is being used to investigate the allegations against Manafort, who has been convicted or pleaded guilty to a series of charges brought by special counsel Robert Mueller.

Trump has refused to rule out the possibility of pardoning Manafort. If state charges were brought, Trump wouldn't be able to intercede in the case.

Manafort's spokesman, Jason Maloni, declined to comment. Danny Frost, a spokesman for the Manhattan district attorney's office, also declined to comment.

Manafort, 69, is scheduled to be sentenced next month on the federal charges. He's facing decades in prison — a virtual life sentence — for a series of crimes ranging from tax fraud to illegal foreign lobbying.

Manafort has been locked up since last June when a federal judge revoked his bail over allegations of witness tampering in the Russia probe.

Last August, a Virginia federal court jury convicted him of eight of 18 charges, including filing false income tax returns and bank fraud.

Manafort went on to plead guilty to conspiracy against the U.S. and conspiracy to obstruct justice to avoid a second trial in Washington. Mueller's office had accused Manafort of crimes connected to illegal lobbying work for pro-Russian politicians in Ukraine.