Democratic Sen. Elissa Slotkin of Michigan is refusing to voluntarily comply with a Justice Department investigation into a video she organized urging U.S. military members to resist “illegal orders” — escalating a dispute that President Donald Trump has publicly pushed.

In letters first obtained by The Associated Press, Slotkin’s lawyer informed U.S. Attorney for the District of Columbia Jeanine Pirro that the senator would not agree to a voluntary interview about the video. Slotkin’s legal team also requested that Pirro preserve all documents related to the matter for “anticipated litigation.”

Slotkin’s lawyer separately wrote to Attorney General Pam Bondi, declining to sit for an FBI interview about the video and urging her to immediately terminate any inquiry.

The refusal marks a potential turning point in the standoff, shifting the burden onto the Justice Department to decide whether it will escalate an investigation into sitting members of Congress or retreat from an inquiry now being openly challenged.