Factbox: What has the testimony to the impeachment inquiry on U.S. President Trump revealed?

FILE PHOTO: U.S. Ambassador to the European Union Gordon Sondland arrives on Capitol Hill before testifying to congress behind closed doors, as part of the impeachment inquiry led by the House Intelligence, House Foreign Affairs and House Oversight and Reform Committees, in Washington, U.S., October 17, 2019. REUTERS/Tom Brenner

WASHINGTON (Reuters) - The U.S. House of Representatives' impeachment inquiry into whether President Donald Trump abused the power of his office to push the Ukrainian president to order an probe of a political rival has heard from a series of administration insiders over the past two weeks.

At the center of the inquiry is a July 25 call in which Trump pressed Ukrainian President Volodymyr Zekenskiy to investigate former vice president Joe Biden, a leading candidate for the 2020 Democratic presidential nomination, and his son Hunter Biden over their ties to a Ukrainian energy company.