House Speaker Nancy Pelosi warned of an escalating investigation into the Trump administration for continuing to block acting Director of National Intelligence Joseph Maguire from turning over a whistleblower complaint.

A whistleblower in the intelligence community filed a complaint alleging that President Trump made a "promise" with the leader of a foreign nation. Additional reporting revealed that the conversation likely took place between Trump and Ukrainian President Volodymyr Zelensky in July about possibly investigating Joe Biden and his son, Hunter.

Pelosi said in a letter to lawmakers Sunday, "If the Administration persists in blocking this whistleblower from disclosing to Congress a serious possible breach of constitutional duties by the President, they will be entering a grave new chapter of lawlessness which will take us into a whole new stage of investigation," according to Axios.

The "new stage" that Pelosi referenced could very well be impeachment, though she has been hesitant to support the idea of an inquiry. A whistleblower complaint could sway her stance, however, and House Intelligence Committee Chairman Adam Schiff said Sunday that impeachment might be necessary, though he conceded that Senate Republicans would never remove Trump from office.

Maguire has refused to hand the complaint over to the congressional committees despite the House Intelligence Committee having issued a subpoena for it.