President Donald Trump’s aides have explored moving some impeachment witnesses on loan to the White House from other agencies, such as Lt. Col. Alexander Vindman, back to their home departments ahead of schedule, according to people familiar with the conversations.

As public hearings bring the officials’ allegations to his television screen, Trump is asking anew how witnesses such as Vindman and Ambassador Bill Taylor came to work for him, people familiar with the matter said. He has suggested again they be dismissed, even as advisers warn him firing them could be viewed as retaliation.

The possible move of officials out of the White House could still be viewed by some as evidence of retribution for their testimony. Trump’s frustration at his own officials comes as he attacks witnesses on Twitter, including during Friday’s public hearing with the ousted ambassador to Ukraine Marie Yovanovitch. Trump appears to have adopted a strategy of maligning the officials, despite some allies encouraging him not to.