WASHINGTON — The Treasury Department has slow-rolled congressional requests from several lawmakers in recent years, raising questions about the department’s relatively quick response to a November request from Republican senators for sensitive financial information on former vice president Joe Biden’s son and the Ukrainian company for which he worked.

Last week, Treasury Secretary Steve Mnuchin testified that the department has complied with congressional requests for suspicious activity reports (SARs) “from both Republicans and Democrats on an equal basis.” Speaking before the Senate Finance Committee, Mnuchin said Treasury had responded to requests from several committees involving “thousands of SARs."

But BuzzFeed News has learned committees investigating Russian election interference and President Donald Trump have waited several months for Treasury’s Financial Crimes Enforcement Network (FinCEN) to turn over records, and some requests from Democrats have yet to be completely fulfilled. In the past, Treasury has been so uncooperative with Congress that senators — both Democrat and Republican — have resorted to holding up Treasury nominees to force the department to turn over documents.

“I’m deeply concerned that Treasury Department oversight requests are being politicized,” Sen. Ron Wyden, the top Democrat on the Finance Committee, said in a statement to BuzzFeed News this week. “I believe that requests should generally be handled in the order they are received, but that’s not how they are being handled under this administration. Requests that further Donald Trump’s interests should not be moved to the front of the line while requests that would shed light on potential wrongdoing by the president and his associates are slow-walked.”

But Treasury disputes that its handling of congressional requests has been politicized.

“The Treasury Department does not process congressional requests for FinCEN records on a partisan basis and has provided FinCEN records in response to requests from both sides of the aisle over the past year,” a Treasury official said in a statement to BuzzFeed News on Thursday. “In fact, the overwhelming majority of those productions have been in response to requests made or joined by Democratic members. Since January 2017, the Treasury Department has provided approximately 60,000 pages of FinCEN records in response to requests from the House and Senate Intelligence Committees — all of which were made or joined by Democratic members.”

On Nov. 15, Republican Sens. Chuck Grassley and Ron Johnson wrote to FinCEN seeking SARs on Hunter Biden, Burisma — the Ukrainian gas company where he served as a board member — and other figures. Grassley, the chair of the Senate Finance Committee, and Johnson, the chair of the Senate Homeland Security and Governmental Affairs Committee, ramped up an investigation into Democrats and Ukraine this fall while Trump’s impeachment on Ukraine-related matters made its way through Congress.

Banks file SARs to FinCEN when they identify transactions that bear the hallmarks of money laundering or other financial crimes. The reports themselves are not evidence of a crime — many are filed on transactions that are perfectly legal — but can be used to support investigations.

Treasury responded to the Republican senators’ request for SARs within about one month, according to a source familiar with the investigation. A spokesperson for Wyden said earlier this month that the department was “rapidly complying” with the request from the Republican senators “and producing ‘evidence’ of questionable origin.” At the time, a spokesperson for Grassley said it was “strange that any senator would complain about receiving responses to oversight requests in a timely manner.”