Special prosecutor Robert Mueller zeroed in on President Donald Trump's business dealings with Deutsche Bank as his investigation into alleged Russian meddling in U.S. elections widens.

Mueller issued a subpoena to Germany's largest lender several weeks ago, forcing the bank to submit documents on its relationship with Trump and his family, according to a person briefed on the matter, who asked not to be identified because the action has not been announced.

"Deutsche Bank always cooperates with investigating authorities in all countries," the lender said in a statement to Bloomberg Tuesday, declining to provide additional information.

Deutsche Bank for months has rebuffed calls by Democratic lawmakers to provide more transparency over the roughly $300 million Trump owed to the bank for his real estate dealings prior to becoming president. Rep. Maxine Waters of California and other Democrats have asked whether the bank's loans to Trump, made years before he ran for president, were in any way connected to Russia. The bank previously rejected those demands, saying sharing client data would be illegal unless it received a formal request to do so. Trump has denied any wrongdoing.

Calls and emails to the White House weren't immediately returned before U.S. office hours. Handelsblatt reported the subpoena earlier Tuesday.

Mueller's investigation -- which is looking into alleged Russian interference into last year's U.S. election and whether Trump's winning campaign assisted in those efforts -- appears to be entering a new phase. Trump's former national security adviser, Michael Flynn, pleaded guilty Friday to lying to FBI agents, becoming the fourth associate of the president ensnared by Mueller's probe. More significantly, he also is providing details to Mueller about the Trump campaign's approach to Flynn's controversial meeting with a Russian envoy during the presidential transition.

Trump's relationship with Deutsche Bank stretches back some two decades and the roughly $300 million he owed to the bank represented nearly half of his outstanding debt, according to a July 2016 analysis by Bloomberg. That figure includes a $170-million loan Trump took out to finish a hotel in Washington. He also has two mortgages against his Trump National Doral Miami resort and a loan against his tower in Chicago.

Deutsche Bank management is ready to share information about the lender's dealings with Trump and is hopeful that doing so will help end the series of inquiries from Democrats, an executive at the bank, who asked not to be identified discussing private deliberations, has previously told 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.

Democratic senators, including Dianne Feinstein, suggested after the Flynn plea that Mueller might be building an obstruction of justice case against Trump. But the Deutsche Bank subpoena may indicate that the special prosecutor is still looking at a wide range of data, including the president's financial dealings.

Mueller's team has also been interviewing White House aides in recent weeks, including former Chief of Staff Reince Priebus, former spokesman Sean Spicer and National Security Council chief of staff Keith Kellogg, according to people familiar with the investigation.

As Mueller's investigation unfolds, Trump has gone on the offensive. Over the weekend on Twitter, he attacked the FBI and Mueller's team and defended some of Flynn's actions. In particular, Trump hailed the news that one of Mueller's aides had been removed from his job over the summer for some anti-Trump text messages.

On Monday, as he left the White House for a trip to Utah, Trump restated his sympathy for Flynn and his assertion that prosecutors should have pursued action against his general election rival, Hillary Clinton.

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Bloomberg contributors: Billy House, Chris Strohm, Ross Larsen, Kathleen Hunter and Alan Katz.