President Trump told reporters at the White House Friday that he "hardly [knows]" EU Ambassador Gordon Sondland, who testified before the House's impeachment inquiry that military aid to Ukraine was conditioned on an investigation into the Biden family's business dealings there.The big picture: Over the past few weeks, the president has continually claimed that he does not know several of the key people involved in the impeachment inquiry.Sondland has been described as "a Trump guy" who donated $1 million to the president's inaugural committee before being named ambassador to the European Union.Lt. Col. Alexander Vindman: On the morning of the impeachment testimony from the National Security Council's top Ukraine expert, Trump said in a tweet, "Why are people that I never even heard of testifying about the [July telephone call between Trump and Ukrainian President Volodymyr Zelensky]." Vindman, who was the first person to directly listen to the call to testify before the House committees, said that acting White House chief of staff Mick Mulvaney "coordinated" a plan to condition a White House meeting for Zelensky on an investigation into the Bidens.Former Ukraine Ambassador Marie Yovanovitch: Trump said earlier this week that he "really [doesn't] know her" on the same day the House released its transcript of her testimony.She testified that Trump's lawyer Rudy Giuliani led a campaign to oust her over unsubstantiated allegations that she badmouthed the president and was seeking to stop Ukraine from opening an investigation into the Bidens.Bill Taylor, the top U.S. diplomat in Ukraine: Trump said in a tweet last month that he doesn't know Taylor — and called the career diplomat a "Never Trumper."Taylor told the impeachment committees that he understood the president to be conditioning the release of military aid on the Ukrainian president's willingness to announce investigations into Trump's political rivals, including the Bidens.Lev Parnas and Igor Fruman: Trump told reporters last month that he didn't know the pair, who are business associates of Rudy Giuliani with ties to Ukraine and are facing charges for allegedly funneling foreign money into Republican political campaigns.Earlier this week, Parnas indicated that he would be willing to comply with the impeachment inquiry. His lawyer said that he "was very upset by President Trump’s plainly false statement that he did not know him."Go deeper: The takeaways from the impeachment inquiry's closed-door phaseOfficial testifies Trump wanted Ukraine's Zelensky to mention "Clinton"Rudy Giuliani lawyers up amid impeachment inquiry