Hill, a Russia expert who reported directly to John Bolton when he was national security adviser, was testifying alongside David Holmes, a top staffer at the U.S. Embassy in Ukraine, before the House Intelligence Committee as part of the impeachment inquiry.
Democrats are seeking information to bolster the case that Trump sought to leverage U.S. military aid to Ukraine and a White House visit by President Volodymyr Zelensky in exchange for investigations of former vice president Joe Biden and his son Hunter Biden, among others.