House lawmakers on Wednesday invited former national security adviser John Bolton to testify next week as part of Democrats’ impeachment investigation into President Donald Trump, according to a report.

The Hill reports Bolton’s closed-door deposition is scheduled for November 7, though the former Trump official’s attorney has not yet confirmed whether his client will attend the closed-door session. Congressional investigators have also requested two current National Security Council staffers, Michael Ellis and lawyer John Eisenberg, to testify November 4.

The development comes after Christopher Anderson, a State Department Foreign Service officer, told lawmakers Wednesday that Bolton told him that President Trump’s personal lawyer, Rudy Giuliani, “was a key voice with the president on Ukraine.” Bolton was also so concerned with Giuliani’s activities in Ukraine that he called the president’s lawyer a “hand grenade who is going to blow everybody up,” according to a former White House aide Fiona Hill.

Bolton was present during a July 25 telephone call between President Trump and Ukrainian President Volodymyr Zelensky, which prompted the impeachment probe following a so-called “whistleblower” complaint. During the call, the president suggested Zelensky look into allegations of corruption against former vice president Joe Biden and his son, Hunter Biden. Both world leaders have denied any pressure was applied to look into the Bidens, and the White House released a transcript of their call as its evidence that no wrongdoing occurred.

“We had a very good conversation with the Ukrainian president. I had another conversation with him also, I think before that, which was the same thing,” President Trump said of the call Monday. “It was nothing.”

The development comes after Charles Kupperman, a former deputy to Bolton, did not appear in Congress Monday to testify as part of the impeachment proceedings, defying a subpoena.

Kupperman had filed a lawsuit asking a judge to resolve the conflicting orders from the congressional subpoena compelling him to testify and the White House ordering not to cooperate. He could receive a contempt citation for failing to appear.

Kupperman’s attorney Charles Cooper said his client wants the courts to resolve the matter before he appears.

“It is not Dr. Kupperman who contests your clients’ constitutional claim,” Cooper wrote in a letter to House intelligence committee senior investigative counsel Daniel Noble. “It is President Trump and every president before him for at least the last half century, who have asserted testimonial immunity for their closest confidential advisers.

“If [the committee’s] position on the merits of this issue are correct, it will prevail in court, and Dr. Kupperman, I assure you again, will comply with the court’s judgment,” he added.

House investigators on the intelligence, foreign affairs, and oversight committees have already deposed multiple witnesses in the inquiry, including U.S. Ambassador to Ukraine William Taylor.

The UPI contributed to this report.