US President Donald Trump fired two key impeachment witnesses Friday, dismissing Lt. Col. Alexander Vindman, the top Ukraine expert at the National Security Council, and US Ambassador to the European Union Gordon Sondland.

An adviser to Trump said the firings of the major impeachment witnesses was meant to send a message that siding against the President will not be tolerated. “Flushing out the pipes,” the adviser told CNN. “It was necessary.”

Vindman was pushed out of his role Friday months earlier than expected, according to a statement from his attorney. Vindman was not slated to leave until July, but had been telling colleagues in recent weeks he would likely leave soon. Sondland said in a statement Friday that he is being recalled from his post.

The dismissals appear to be retribution for Vindman and Sondland’s explosive testimonies to the House impeachment probe late last year, both of which were done under subpoena. The duo gave some of the most damning testimony to House impeachment investigators during last fall’s public hearings and quickly became targets for Trump’s supporters, both inside and outside of government.

The firings come two days after the Senate voted to acquit Trump on two articles of impeachment in votes that were largely along party lines, as reported by CNN.