The US Department of Justice is wading into a legal battle to quash subpoenas from the Manhattan District Attorney’s Office seeking President Trump’s corporate and personal tax returns.

In a brief letter to Manhattan federal Judge Victor Marrero filed late Monday, the chief of the US Attorney for the Southern District’s civil division, Jeffrey Oestericher, said his office would file a more substantive submission by the close of business Wednesday.

Federal prosecutors’ motives for joining the case remain unclear, though they hinted in a letter last week that Trump’s lawsuit “raises a number of significant constitutional issues that potentially implicate the interests of the United States.”

Monday’s letter follows an attempt by DA Cyrus Vance’s office to subpoena Trump’s accounting firm, Mazars USA, demanding personal and business records of the president and other third parties dating back to 2011.

Trump subsequently sued to block the subpoena, saying the request was unconstitutional and he was “immune” from inquiry while in office.

Both sides agreed to temporarily stay the subpoenas last week, following testy arguments before Marrero in which Trump lawyers characterized Vance’s grand jury inquiry as presidential harassment.

Vance’s office argued its subpoena was valid, and that the dispute belonged before a state judge and not a federal one.