Former US national security adviser John Bolton has failed to show up for an interview with impeachment investigators.
His move makes it unlikely that he will provide any testimony to the House about President Donald Trump’s handling of Ukraine.
An attorney for Mr Bolton, Charles Cooper, said his client had not received a subpoena. Mr Cooper had previously said Mr Bolton would not appear without one.
A House intelligence committee official said the panel has no interest in engaging in a drawn-out court fight over a subpoena for Mr Bolton and will simply add the White House’s instructions against testifying as evidence of the President’s obstruction of Congress.
Even as Mr Bolton was a no-show, an aide to Vice President Mike Pence came to the Capitol to speak to impeachment investigators .
Jennifer Williams, a career foreign service official detailed to Mr Pence’s office from the State Department, was subpoenaed to appear.
She is one of several White House aides who were listening in on a July phone call between Mr Trump and Ukraine President Volodymyr Zelenskiy in which Mr Trump asked the new leader to investigate Democrats, according to an administration official.
That call is at the centre of the Democrats’ impeachment probe.
Speaking to reporters in New Hampshire, Mr Pence stood by Mr Trump and said if Americans read the administration’s rough transcript of the call they will find “there was no quid pro quo, the president did nothing wrong”.
Mr Pence called the impeachment inquiry a “disgrace”.
Investigators are wrapping up the private interviews as they prepare to start public hearings next week.
House Intelligence Committee chairman Adam Schiff announced on Wednesday that three State Department witnesses will appear in two hearings next Wednesday and Friday – top Ukraine diplomat William Taylor, career department official George Kent and Marie Yovanovitch, the former US ambassador to Ukraine.
Ms Yovanovitch was ousted in May on Mr Trump’s orders and Mr Taylor replaced her. Both have testified about their concerns over the administration’s policy on Ukraine.
Democrats scheduled 13 witnesses to testify behind closed doors this week, but so far only Ms Williams and another State Department employee, David Hale, have shown up. Mr Trump has directed his employees not to cooperate with the probe.
Mr Trump asked Mr Zelenskiy to investigate political rival Joe Biden and his family and also Ukraine’s role in the 2016 US presidential election.
Though Mr Trump has said there was no “quid pro quo,” several of the witnesses, including Mr Taylor, have testified that it was their understanding that Ukraine would not receive military assistance or a coveted Oval Office visit until it met the president’s demands.