A week marked by disclosures that President Donald Trump may have asked the director of the FBI to back off a politically charged investigation and offered highly classified information to Russian diplomats during an Oval Office meeting has dominated the cable news channels and led to the first cautious mentions of the word "impeachment" by a small number of lawmakers on Capitol Hill.

But as Trump's poll numbers slide amid a perception that his presidency is failing, a significant development in a lesser-known investigation threatens to raise more uncomfortable questions for the president at an inconvenient time.

Federal investigators on Tuesday issued subpoenas for records from former Trump campaign chairman Paul Manafort, who is already under a cloud for his links to Kremlin interests. Manafort apparently took out a $3.5 million mortgage on his Hamptons home shortly after leaving the campaign. The loan was made by a firm with ties to both Trump and a Ukrainian-American real estate magnate, and it was not accompanied by paperwork that normally tracks whether the mortgage is repaid, prompting inquiries about whether the transaction had anything to do with Manafort's role in the campaign.

Investigators have demanded records pertaining to the mortgage, according to NBC News, which first reported the story.

The loan was apparently granted by a subsidiary of a company called Spruce Capital, founded by Joshua Crane, who has worked with Trump on real estate transactions. The firm is also in part funded by Alexander Rovt, a Ukrainian-American real estate tycoon. The firm never filed a mortgage notice with the county – a decision that does not appear to be illegal, but is critical for showing whether a mortgage has been paid off.

Manafort has been under intense scrutiny for receiving as much as $1.2 million in payments from a pro-Russia political party in Ukraine. He is also under state investigation by New York Attorney General Eric Schneiderman's office, which said it is examining his real estate transactions.

Manafort served as campaign chairman for less than five months in 2016. A longtime Republican political operative, Manafort was brought in last April to bring order to what had been a chaotic campaign, but he was out by that August, brought down by the campaign's internal anarchy, negative press, and a flood of reports about his connections with Russian-backed politicians in Ukraine.

He has since been swept up in what appear to be multiple investigations, including the FBI's probe into Russian interference in the 2016 election and its ties to members of the Trump team.

In the course of three days in March, a lawmaker in Ukraine released documents accusing Manafort of laundering $750,000 in October 2009 on behalf of Ukraine's Russia-backed ex-president. The next day, the Associated Press reported that Manafort secretly worked for Russian billionaire to advance Russian interests from 2006 until at least 2009. And the day after, ABC News reported that Manafort's financial records in Cyprus were turned over to the Treasury Department in connection with a 2014 case.

By April, Manafort announced he would register as a foreign agent, an action and office referred to as FARA. Spokesman Jason Maloni told NBC News that Manafort is working with authorities.

"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 in April to The New York Times, Manafort maintained that there is nothing untoward about payments that he's received.

“Mr. Manafort has always denied that he ever received any cash payments for his work and has consistently maintained that he received all of his payments, for services rendered, through wire transfers conducted through the international banking system,” the statement said.