Washington (CNN) As President Donald Trump has ramped up his attacks on Lt. Col. Alexander Vindman, the National Security Council's top Ukraine expert has continued to report to work, CNN has learned.

Behind closed doors, Vindman testified before House impeachment investigators for more than 10 hours last week. He was the first witness who listened to the July call with the leader of Ukraine to go under oath. He testified that he was deeply troubled by it , reported those concerns to the top National Security Council lawyer within hours and also said some of the changes he tried to make to the since-published transcript were left out, though he didn't say why.

Since his appearance, Trump has attempted to sow doubt about Vindman's political ties, calling him a "Never Trumper witness" and implying more information would be revealed about him in the coming days.

"Well, you'll be seeing very soon what comes out and then you can ask the question in a different way," Trump said Sunday without elaborating.

He has declined to provide any evidence to back up his claims and the White House has not responded to a request for comment.

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