Deutsche Bank’s disclosure on Tuesday that it has tax returns related to President Trump’s family or business set off a frenzy of speculation about what those materials might reveal.

But a trove of other data and documents that his longtime lender is sitting on might prove more revelatory to investigators digging into Mr. Trump’s finances. That includes records of how Mr. Trump made his money, whom he has partnered with, the terms of his extensive borrowings and what transactions he has engaged in with Russians or other foreign nationals.

For nearly two decades , Deutsche Bank was the only mainstream financial institution consistently willing to do business with Mr. Trump, who had a long record of defaulting on loans. The bank over the years collected reams of his personal and corporate information.

Two congressional committees have subpoenaed Deutsche Bank for a vast array of records related to Mr. Trump — including any tax returns since 2010. The investigators are hoping the materials will shed light not only on the president’s finances but also on any links he has had to foreign governments and whether he or his companies were involved in any illegal activity, such as money laundering for people overseas.