Donald Trump's former deputy national security adviser Charles Kupperman, aide to former national security adviser John Bolton, did not appear for a scheduled deposition Monday morning in the impeachment inquiry into Trump. House Intelligence Committee Chairman Adam Schiff said that Kupperman’s failure to appear "is deeply regrettable" and may warrant contempt proceedings against him. His refusal, Schiff said, amounts to "additional evidence of obstruction" of the inquiry.

There's a possibility that Kupperman will eventually show. His attorneys have filed a lawsuit asking a federal judge whether he is obliged to testify, and until a judge rules he has to be there, he will not appear, his lawyers told the Intelligence Committee in a letter Sunday. "If [the committee's] position on the merits of this issue is correct, it will prevail in court, and Dr. Kupperman, I assure you again, will comply with the Court's judgment," his lawyer wrote to a Democratic attorney for the committee. The White House is exerting executive privilege to prevent his testimony, even though he's no longer in his official position, and the House has subpoenaed him, so he's appealing to a judge to rule on what he has to do. In return, the committee chairs have warned him that, as a private citizen, he has a legal obligation to appear, and not showing could result in a contempt of Congress citation.

"Notwithstanding this attempted obstruction, the duly authorized subpoena remains in full force and Dr. Kupperman remains legally obligated to appear for the deposition on Monday," the three chairmen responded in their own letter. Kupperman was reportedly present for the infamous July 25 call between Trump and Ukrainian President Volodymyr Zelensky. Meanwhile, his boss, former national security adviser John Bolton, has been in talks with the three House committees, negotiating an appearance.