Among the many mysteries surrounding Donald Trump’s finances as a real-estate mogul—and the conflicts of interest that might be revealed by his tax returns, were he ever to release them—is his long history of debt. The issue is not merely what Trump owes, but who he owes. As critics noted on the campaign trail, Trump’s habit of reneging on contracts and suing his lenders meant that virtually nobody on Wall Street wanted to work with him, with one exception: Deutsche Bank, which had loaned him hundreds of millions of dollars when no one else would, even after he sued the firm. Now, investigators probing the ties between the Trump campaign and Russia are wondering why—and they’re beginning to take a closer look at the president’s accounts with his favorite bank, which also happens to have strong ties to Russia itself.

The New York Times reports that banking regulators are currently “reviewing hundreds of millions of dollars in loans made to Mr. Trump’s businesses through Deutsche Bank’s private wealth management unit . . . to [see] if the loans might expose the bank to heightened risk.” Meanwhile, the Guardian reports that executives at Deutsche are “expecting that the bank will soon be receiving subpoenas or other requests for information from Robert Mueller,” and that the special counsel’s investigative team and the bank have “already established informal contact in connection to the federal investigation.”

There’s certainly plenty to look into. Over the last 20 years, Trump has received more than $4 billion in loan commitments and potential bond offerings from the German lender, despite suing the company in 2008 when he fell behind on payments on the $640 million loan he was given to build Trump International Hotel & Tower in Chicago. Incredibly, in order to avoid paying the $40 million he had personally guaranteed, Trump and his lawyer argued that “Deutsche Bank is one of the banks primarily responsible for the economic dysfunction we are currently facing”—i.e. the global financial crisis—and therefore it should pay him $3 billion in damages under the extraordinary event clause in his contract. Naturally, the bank countersued, calling the real-estate developer’s claim “classic Trump.” In the end, after threatening to take his name off the building if he wasn’t granted more time to pay, the bank gave Trump extra time; when he did pay the money he owed to the firm’s real-estate lending division, it was with another loan he got from Deutsche’s wealth-management unit. Trump subsequently moved his business from the real estate group to the private wealth management group, where, according to the Times, “executives were more willing to deal with him.” One of those executives was Rosemary Vrablic, who has helped finance three Trump properties over the last six years, lending $300 million in the process. That amount is “somewhat unusual by Wall Street standards,” former and current Deutsche Bank executives and wealth managers at other firms on Wall Street told the Times.

In addition to Donald, Ivanka Trump is also said to be a Deutsche Bank client, as is Jared Kushner and his mother, who, per the Times, have “an unsecured line of credit from Deutsche Bank, valued at up to $25 million.” In addition, the Kushner family business, Kushner Companies, got a $285 million loan from the bank last year. And because the Kushners and Trumps have never shied away from conflicts of interest, in 2013, Kushner reportedly “ordered up a glowing profile of [Vrablic] in the real estate magazine he owned,” with a disclosure about their connection at the very end of the piece.