Washington (CNN) President Donald Trump's refusal to sign an August memo releasing funds for Ukraine offers the latest evidence that the money was paused not as part of a security review -- as administration officials previously claimed -- but instead solely at the discretion of the President.

Officials at the White House's Office of Management and Budget have said that Trump tapped Defense Secretary Mark Esper and then-national security adviser John Bolton to review the money. Once the review was completed, administration officials said the funds were released.

But witnesses in the impeachment inquiry and other sources familiar with the matter contradict that version of events. Bolton was prepared to release the money in August. Esper was inclined to do so even earlier. And still, the money languished until nearly mid-September.

Rather than being held up for a security review, it appears the funds were being withheld at Trump's demand as he pressed officials in Ukraine to investigate the 2016 US presidential election and his potential 2020 presidential rival, former Vice President Joe Biden. The President's rationale for pausing the aid, and then for releasing it, are both of interest to impeachment investigators. Multiple witnesses, including the National Security Council's top Ukraine expert, Lt. Col. Alexander Vindman, have testified that they believed Trump was withholding money from Ukraine in exchange for political investigations

The White House did not respond to a request for comment. It has previously said that the President did nothing wrong, that he simply wanted to ensure Ukraine was cracking down on corruption and that US allies were doing their fair share to support Ukraine.

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