WATCH: four witnesses to testify at back-to-back public impeachment hearings
The House's impeachment hearing marathon is underway.
Impeachment investigators will hear publicly from nine witnesses at five impeachment hearings over the next three days as Democrats race to collect public testimony this week about President Donald Trump's role in pushing Ukraine to open investigations and the withholding of US security aid and a one-on-one meeting.
Watch ABC News' live coverage:
The National Security Council's top Ukraine expert Lt. Col. Alexander Vindman and Jennifer Williams, an aide to Vice President Mike Pence, are testifying Tuesday morning, while former US special envoy to Ukraine Kurt Volker and Tim Morrison, a former a top Russia and Europe adviser on the National Security Council, appear together in the afternoon.