Federal investigations into former Trump campaign chairman Paul Manafort appear to be ramping up, as NBC News reported Tuesday that the feds have subpoenaed records related to a $3.5 million mortgage that Manafort took out on his country home in in the Hamptons.

An unnamed source close to the matter told NBC News that Manafort never filed a document detailing how he would pay back the multi-million dollar loan, and did not pay roughly $36,000 in taxes on it.

Real estate experts told the network that the omission was “highly unusual” and “totally ill-advised,” but not illegal.

Manafort’s opaque real estate dealings are reported to be at the core of multiple probes officials at the city, state and federal level. New York state Attorney General Eric Schneiderman and Manhattan District Attorney Cyrus Vance Jr. have opened preliminary investigations into the millions of dollars of property he owns, according to reports in the Wall Street Journal and Bloomberg News.

The Journal reported that the Justice Department has also requested that Citizens Financial Group Inc. turn over Manafort’s banking records as part of its investigation into whether Trump campaign officials colluded with Russians working to sway the U.S. presidential election.

Manafort spokesperson Jason Maloni told TPM that Manafort repaid the loan, which he took out through a shell company, in late November or early December.

“Mr. Manafort has not been contacted by any authorities other than the United States Congress and officials responsible for FARA guidance, and he is cooperating with those inquiries,” Maloni said in a statement.

NBC News reported that Manafort created a holding company calling Summerbreeze LLC on Aug. 19, the same day he resigned from the Trump campaign. Summerbreeze took out a $3.5 million loan on Manafort’s home in Bridgehampton, a wealthy Long Island beach town, several weeks later, but never filed the mortgage notice with Suffolk County, according to the report.

WNYC reported in March that Manafort had a pattern of using shell companies to make all-cash purchases of expensive real estate properties.

This post has been updated.