David Holmes, a State Department official working at the U.S. Embassy in Ukraine, will testify publicly in an impeachment hearing on Thursday alongside President Trump's former Russia adviser Fiona Hill, according to House officials.Why it matters: In a closed-door deposition, Holmes testified that he overheard a phone call between EU Ambassador Gordon Sondland and Trump on July 26, the day after the infamous call between Trump and Ukrainian President Volodymyr Zelensky. Holmes claimed he heard Trump ask Sondland, "So, he's gonna do the investigation?" to which Sondland responded, "He's gonna do it," adding that Zelensky would do "anything you ask him to."Read Holmes' opening statement in his deposition Go deeper: The full schedule for this week's impeachment hearings