Breaking from Bloomberg News:

News of the Deutsche Bank subpoena may escalate Trump’s vitriol toward investigators, which over the past week has included publicly attacking the Justice Department and the Federal Bureau of Investigation, whose reputation he declared to be “in tatters.”

In July, Trump said in an interview with the New York Times that if Mueller examined his family’s finances beyond any relationship with Russia he’d consider it "a violation." Mueller’s investigation had expanded to examine a broad range of transactions involving the president’s businesses, including dealings by his son-in-law, Jared Kushner, and Commerce Secretary Wilbur Ross, a person familiar with the probe told Bloomberg News after the publication of the Times interview.

Mueller issued the subpoena weeks ago, according to the story. It indicates that Mueller's probe still extends beyond possible obstruction of justice.

Democratic lawmakers have unsuccessfully requested those Deutsche Bank records for months. Trump owes them $300 million for his past real estate dealings.

Bloomberg reports that Trump has had a relationship with Deutsche Bank for two decades and the estimated $300 million owed to the bank represents nearly half of his outstanding debt, according to a July 2016 analysis by Bloomberg.

Why does Mueller want the information? We can't know for sure, but Deutsche Bank has a very strong presence in Russia. Maybe there's a connection!