WASHINGTON (AP) — President Donald Trump suggested Tuesday that the Pentagon should review the conduct of a former White House national security aide who played a central role in the Democrats' impeachment case and potentially consider disciplinary action against him.

Army Lt. Col. Alex Vindman, who until last week was detailed by the Pentagon to the White House, testified before the House impeachment panel that Trump inappropriately pushed Ukraine President Volodymyr Zelenskiy to investigate former Vice President Joe Biden and his son Hunter.

Vindman was ousted from his job on the White House National Security Council on Friday, just two days after the Senate acquitted Trump on abuse of power and obstruction of justice charges. He is to be reassigned by the Pentagon. His twin brother, Lt. Col. Yevgeny Vindman, who worked as an ethics lawyer at the NSC, also was ousted from his job and was re-assigned to the Army General Counsel's Office.

Trump has already been sharply criticized for pushing out Alexander Vindman and others who crossed him in the impeachment hearings.

Asked whether he wanted to see Vindman face discipline, Trump said military officials "can handle him any way they want" but also suggested that he expected commanders to review the decorated, 20-year officer's conduct.

"That's going to be up the military," Trump said. "We'll have to see. They are certainly, I imagine, going to take a look at that."

Robert O'Brien, Trump's national security adviser, said later Tuesday that Alexander Vindman was not asked to leave his White House job in retaliation for his testimony in the Democrats' impeachment case.

"There 's absolutely no retaliation with respect to the Vindmans as far as impeachment goes," O'Brien said at an event hosted by the Atlantic Council. "But the president is entitled to a staff that he has confidence in and that he believes will execute his policy."

O'Brien said it was his decision and that the president did not ask him to remove them.

"They were not retaliated against," O'Brien said.

"Their services were no longer needed. ... The hiring and the decisions on personnel are made by the National Security Council. Ultimately the buck stops with me, but there is a chief of staff, we have a deputy and we have lawyers who are involved in every one of those decisions," he said. "So those were my decisions and I stand by them."

Noting that both are lieutenant colonels in the Army, O'Brien suggested that the Vindman twins were not acting to implement the president's policy decisions.

"The policy of the United States is formed and decided by the president of the United States," he said. "We're not some banana republic where lieutenant colonels get together and decide what the policy is."

Defense Secretary Mark Esper was asked earlier what the Pentagon would do to ensure that the former NSC staffers face no retribution.

"We protect all of our service members from retribution or anything like that," Esper said. "We've already addressed that in policy and other means."

Alexander Vindman's attorney, David Pressman, did not immediately respond to a request for comment.

Earlier, Pressman said that telling the truth cost his client his "job, his career, and his privacy."

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Associated Press writers Kevin Freking and Deb Riechmann contributed to this report.