Of all the investigations into his dubious behavior, the one that most sets Donald Trump’s hair on fire involves Deutsche Bank. The German lender is famous for being the only institution that would loan the ex-real-estate developer money while everyone else treated him like the pariah he was, and presumably knows more about his finances than anyone outside the Trump Organization. Earlier this week, the president, his company, and his three eldest children sued Deutsche (and Capital One) in an effort to stop the banks from responding to congressional subpoenas, and a new report from Bloomberg hints at why Trump seems particularly antsy about the idea of Germans telling Democrats everything they know: in addition to decades’ worth of information concerning his financial transactions, some people inside the firm have actually seen his elusive tax returns.

Per Bloomberg, in 2012, the Trump Organization was trying to obtain loans from the bank for the Trump National Doral Golf Club in Florida and the Trump International Hotel & Tower in Chicago. In the wake of the financial crisis, Trump had defaulted on a loan from the bank for the Chicago property, and in an absurd move even for him, filed a lawsuit against Deutsche, claiming it had caused the global financial crisis and “engaged in predatory lending against him.” (The bank promptly countersued, and the case was eventually settled.) Having been burned by the bankruptcy king in the past, Deutsche initially balked at the idea of lending the Trump Organization its requested $100 million, at which point Ivanka Trump, the point person on the discussions, suggested her father would guarantee the loans with his personal assets.

That got Deutsche listening, but it insisted on performing due diligence first. And because Trump’s tax returns apparently contain information on par with the Pentagon Papers, their appraisal was carried out in a slightly unorthodox fashion. According to Bloomberg, the bank sent a team to the office of the Trump Organization’s chief financial officer, Allen Weisselberg, who “allowed the bankers to see relevant parts of Trump’s tax returns and take notes.” However, the bankers “weren’t allowed to make copies” of any of the records to take with them. It’s almost as though the returns contain information that might make Trump look bad!

While Democrats have requested that the Internal Revenue Service turn over the president’s filings, thus far their efforts have been rebuffed by loyal stooge Steven Mnuchin, who appears prepared to swallow six years’ worth of tax documents before letting lawmakers get their hands on them. As Bloomberg notes, “even if the I.R.S. is not forthcoming with the presidential tax returns, House Democrats might get key information from bankers’ notes if their subpoena of Deutsche Bank is ultimately successful.”

Last month, a lengthy story by reporter David Enrich shed some light on the long, occasionally fraught relationship between Trump and the German bank, whose “ravenous appetite for risk” translated to happily lending money to the former real-estate developer when no one else on Wall Street would, and going along with his vast financial lies. For instance, in 2004, Trump asked the bank’s commercial real-estate group to lend him more than $500 million to build his 92-story skyscraper in Chicago; it did, but not before employees concluded he was majorly inflating his net worth, and were told he’d “worked with people in the construction industry connected to organized crime.” Ten years later, when Trump was trying to buy the Buffalo Bills and needed to prove he could pull off a transaction that could exceed $1 billion, the bank agreed to vouch that his net worth was $8.7 billion—a number his former fixer, Michael Cohen, told lawmakers was a wild exaggeration.