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(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.

Despite the attacks, Vindman has been seen in his office at the Eisenhower Executive Office Building next door to the White House every day since he testified. The other National Security Council official who testified, Tim Morrison, announced last week that he would leave the administration in the coming days.

After Vindman testified, several of the President’s allies came under fire for implying he was loyal to Ukraine, where he was born but left for the United States when he was three. Vindman is an Army officer who awarded a Purple Heart after being wounded by a roadside bomb in Iraq, and he still carries shrapnel from the attack in his body.

In recent days, Trump has also escalated his attacks on the whistleblower at the center of the impeachment inquiry, implying the person is politically motivated and urging reporters to disclose their identity. Trump asserted Monday that the offer for the unnamed individual to submit written answers to Republican lawmakers – which the person’s attorney offered over the weekend – would not be sufficient.

“The Whistleblower gave false information & dealt with corrupt politician Schiff,” Trump tweeted. “He must be brought forward to testify. Written answers not acceptable! Where is the 2nd Whistleblower? He disappeared after I released the transcript. Does he even exist? Where is the informant? Con!”

The President had a different view on written answers when he submitted his own to the former special counsel Robert Mueller as he was conducting his investigation into Russian interference in the 2016 election. Trump refused to sit for an interview with Mueller and only answered questions about his actions before he took office.

Trump’s attempts to undermine the whistleblower’s credibility come as House investigators have spoken to roughly a dozen witnesses about his efforts to pressure the government of Ukraine.