There have long been whispers about Trump’s property sales to wealthy Russians, and his hundreds of millions in credit from Deutsche Bank, made just as the bank was conducting a massive Russian money-laundering operation.

This week, three statements added fuel to speculation that Trump’s financial history may be the most sensitive thing unearthed by investigations into his presidential campaign:

First, in a complaint (paywall) about their allegedly biased treatment by Congressional investigations into Trump’s ties to Russia, the owners of investigations firm Fusion GPS (who commissioned the infamous Steele dossier) said the Republican-led investigation committees had ignored their suggestion of looking into his funding from Deutsche and others:

We found widespread evidence that Mr. Trump and his organization had worked with a wide array of dubious Russians in arrangements that often raised questions about money laundering…those deals don’t seem to interest Congress.

Representative Adam Schiff, the top Democrat on the House Intelligence Committee then told NPR that he gave credence to Fusion GPS’s allegations:

It is all too true. And we have urged the committee to request and subpoena if necessary—as I think it would be—records from Deutsche Bank. They’ve refused to do so. There are credible allegations that the Russians may have laundered money through the Trump organizations or Trump properties, and that might be leverage that could be used over the president of the United States. I think it would be negligent for us not to either show that that’s true or not true.

Schiff is unable to direct how the committee uses its subpoena powers, since it is chaired by Republican Devin Nunes.

To cap it all, Michael Wolff’s notorious book Fire and Fury quotes former Trump right hand man Steve Bannon saying that Mueller’s investigation will “crack Don Junior like an egg on national TV.” The quote was first made public in a Guardian excerpt.

You realise where this is going…This is all about money laundering. Mueller chose [senior prosecutor Andrew] Weissmann first and he is a money-laundering guy. Their path to fucking Trump goes right through Paul Manafort, Don Jr and Jared Kushner…It’s as plain as a hair on your face…It goes through Deutsche Bank and all the Kushner shit. The Kushner shit is greasy. They’re going to go right through that. They’re going to roll those two guys up and say play me or trade me.

Why aren’t some investigations probing Trump’s finances?

Democrats suggest that Republicans in the various committees tasked with looking into Russian meddling in the 2016 election have chosen not to look too closely at Trump’s finances for political reasons. The House Intelligence Committee, for example, has become so partisan that the Democrat and Republican members plan to issue separate reports. Republicans reportedly say that after exhaustive digging they have found little evidence of collusion, while Democrats believe important areas remain unexplored.

However, special counsel Robert Mueller’s separate investigation seems to already be tracking the money trail.

In December, German newspaper Handelsblatt and other US outlets reported that Mueller had subpoenaed Deutsche Bank for information on dealings “linked to the Trumps.” Trump’s legal team have denied that a subpoena has been issued, but the publications stood by their stories (the Wall Street Journal corrected (paywall) its report to say it related to “people or entities close to” Trump, rather than Trump or his family directly.)

Should Mueller find evidence of criminal activity by the Trump family that isn’t related to the campaign, he probably has the legal basis to investigate it. Deputy attorney general Rod Rosenstein authorized him (pdf) to probe “any links and/or coordination” between Trump’s campaign and the Russian government, and, crucially, “any matters that arose or may arise directly from the investigation.” Mueller has already used the latter power to charge ex-campaign chairman Paul Manafort and his deputy Rick Gates with a dozen crimes including money laundering. Rosenstein has to approve all Mueller’s major steps and presumably signed off on this, so he clearly also thinks such activity is within the probe’s purview.

Manafort has responded by suing (paywall) both Mueller and Rosenstein, arguing that Rosenstein gave the special counsel too much power. Legal experts say the suit is unlikely to succeed.

Why exactly are Trump’s Deutsche Bank loans suspicious?

Trump has insisted he has “no deals” with Russia, but Russians own at least $98.4 million of Trump properties, according to Reuters. His son Eric reportedly told a journalist in 2013/2014 that, “We don’t rely on American banks. We have all the funding we need out of Russia.” (Eric later denied saying this.)

All this has got the likes of congresswoman Maxine Waters, the top Democrat on the House Financial Services Committee, asking whether any of Deutsche Bank’s loans to Trump actually originate in Russia. The former head of Britain’s MI6 intelligence services, Richard Dearlove, has similarly said, “‘What lingers for Trump may be what deals—on what terms—he did after the financial crisis of 2008 to borrow Russian money when others in the West apparently would not lend to him.” Dearlove didn’t provide any evidence to back up the suggestion.

Trump obtained his loans in extraordinary fashion. In the mid-2000s, he had burned so many creditors that Deutsche was the only major player willing to lend to him. However, when the 2008 financial crisis hit, the German bank was next in line, as Guardian reporter Luke Harding reports in his book Collusion (excerpted by Newsweek). First, Trump defaulted on a payment on a $640 million loan for his Chicago hotel and the bank sued him. Instead of paying it, he countersued Deutsche for $3 billion, claiming they owed him damages for helping cause the Great Recession. Eventually, he repaid the bank by getting a loan in 2010 from Deutsche themselves—this time from their wealth management division, rather than the presumably infuriated real estate team.

At the same time, Deutsche’s Moscow branch was carrying out a $10 billion money-laundering scheme to get cash out of Russia into the West via offshore shell companies, according to New York State’s Department for Financial Services. New York State and the UK government have fined the bank a combined $630 million over the scheme. The Russians whose money was being laundered have never been identified, but Deutsche’s New York office was a major beneficiary of the scheme—the very office that was making large loans to Trump. When Trump was elected, he was around $300 million (paywall) in debt to the bank.

It’s not just Trump who is tied to Deutsche. The Kushner deal that Bannon is calling “greasy” in Fire and Fury may be a $285 million loan made to the president’s son-in-law in October 2016, just before the election. Federal prosecutors in Brooklyn requested details (paywall) from Deutsche Bank on the loan in December 2017.

Another possible reason for Mueller’s subpoena was, according to a Reuters source, to find out if Deutsche had sold any of Trump’s loans to Russian state development bank VEB, which is seen as tied to Russian intelligence services. Kushner met with (paywall) VEB head Sergei Gorkov in Dec. 2016 and failed to disclose the meeting.

Why does all this matter?

The other reported main threads of the Mueller probe—collusion and obstruction of justice—are vague and tricky to prove. Collusion is not a federal crime, while charging someone with obstruction of justice demands that you prove a person’s criminal intent (paywall). Both rely on accounts of incidents reported by people with agendas.

That all makes it very difficult to find a smoking gun—especially given that Mueller almost certainly won’t actually indict Trump, instead leaving it to Congress to decide whether he has committed impeachable offenses. And the fact that it would be partisan politicians trying Trump, rather than a judge, means a build-up of circumstantial evidence are even less likely to carry water. Trump’s wild, confused, and often contradictory actions and statements can—and have been—explained away in all sorts of ways.

With plain old money laundering, however, the crime is impossible to deny or overlook—it’s all there for you on paper. As sitting president, Trump almost certainly wouldn’t be criminally indicted if such a scheme were demonstrated, but could members of Congress really bring themselves to support a proven fraudster?