A top Pentagon official involved in the controversial aid deal to Ukraine that resulted in President Donald Trump’s impeachment announced his resignation at Trump’s request on Wednesday.

In his resignation letter, Rood noted that Defense Secretary Mark Esper had asked for his resignation on behalf of Trump.

John Rood, the Pentagon’s under secretary for policy, will leave his post at the end of the month, said Pentagon press secretary Alyssa Farah. Rood’s is the latest in a series of departures of officials who had at least some involvement in investigations involving the Trump administration. Announcements have been made in the weeks since the Republican Senate acquitted Trump of impeachment charges.

Trump and Esper thanked Rood for his service following the announcement. Rood had also worked on issues such as nuclear deterrence, NATO and the National Defense Strategy, which sets threats from China and Russia as the Pentagon’s top priorities.

“I wish him all the best in his future endeavors,” Esper said in a statement.

Rood had been involved in the Pentagon’s effort to send $250 million in military aid to Ukraine, the issue that resulted in President Donald Trump’s impeachment. Rood had certified that Ukraine had met anti-corruption requirements, deeming it eligible to receive the assistance. That certification came before Trump’s July 25 phone call with Ukraine’s president, Volodymyr Zelensky, during which he mentioned a “favor.”

The House impeached Trump for conditioning military aid to Ukraine and a White House visit for Zelensky in exchange for Ukraine opening an investigation into former Vice President Joe Biden.

Two U.S. officials, who were not authorized to speak publicly, said Rood’s decision to resign was not related to the Ukrainian aid.

In the highest profile departures, Trump dismissed two officials who testified during the House impeachment inquiry: Army Lt. Col. Alexander Vindman, a Ukraine expert with the National Security Council, and Gordon Sondland, his ambassador to the European Union, from his post in Brussels.

In addition, Jessie Liu resigned after the Trump administration pulled her nomination to a top Treasury Department post.

Liu served as U.S. attorney for the District of Columbia when her office prosecuted Roger Stone and Paul Manafort. Both are former advisers to Trump, who has criticized the handling of both cases.

As the Democratic presidential candidates hit the debate stage in Nevada Wednesday night, they’ll be joined for the first time by billionaire Mike Bloomberg, who has until now managed to avoid face to face conflict with his rivals. (Feb. 18)