(CNN) The National Security Council's top Ukraine expert told House impeachment investigators on Tuesday that he tried to make changes to the White House's rough transcript of the July phone call between President Donald Trump and Ukraine's President, including that Trump mentioned tapes of former Vice President Joe Biden, according to a source familiar with the matter.

Lt. Col. Alexander Vindman testified that one example of his attempts to change the transcript was to include Trump telling Ukraine President Volodymyr Zelensky there were tapes of Biden, which The New York Times reported occurred where there's an ellipsis in the transcript that was released. The change was not made. The assertion that some portion of the conversation was replaced by an ellipsis contradicts the White House's statement in September that the ellipses in the transcript did not represent missing words or phrases. It also contradicts the President who has insisted the transcript the White House released was an exact depiction of the call, even though the memo itself describes it as rough.

Vindman also said that he would have edited the transcript to specifically show that Zelensky mentioned Burisma -- the company that hired Hunter Biden -- rather than just "the company," according to sources.

"He or she will look into the situation, specifically to the company that you mentioned in this issue," the rough transcript cites Zelensky as saying.

Vindman's testimony that some specific details were left out of the rough transcript adds further insight about how the White House handled the call and Democrats' concerns that the Trump administration engaged in a coverup.

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