Federal investigators are intensifying their probe of President Trump’s former campaign chairman Paul Manafort, issuing subpoenas for records related to the $3.5 million mortgage he took out on his home in the Hamptons, NBC News reported Wednesday.

Manafort never filed the mortgage document disclosing how he would pay back the loan, NBC reported, citing a source close to the matter. He also never paid taxes on the loan, amounting to as much as $36,000.

A spokesman for Manafort told NBC that the former Trump campaign leader had repaid the mortgage in December, but acknowledged that the repayment paperwork had not been filed until later.

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Manafort took out the loan through a shell company, and neither his name nor his signature appear on any of the publicly available mortgage paperwork. The shell company, Summerbreeze LLC, was created on August 19 — the day that Manafort resigned from the Trump campaign.

Manafort’s real estate dealings are the subject of city, state and federal investigations. New York State Attorney General Eric Schneiderman and Manhattan District Attorney Cyrus Vance Jr. are both conducting preliminary probes. 

Manafort, a lobbyist and political consultant, resigned as Trump’s campaign chairman in August amid revelations of his past work with a pro-Russian political party in Ukraine. 

Manafort came under scrutiny once again when the Associated Press reported that he had sent Russian aluminum magnate Oleg Deripaska a proposal in 2005 offering services meant to benefit the government of Russian President Vladimir Putin.

The former campaign chairman is also considered a key figure in ongoing federal investigations into the Trump campaign’s possible ties to Moscow, although he has denied any improper contact with Russian officials.