Fiona Hill, the former National Security Council senior director for Europe and Russia, and David Holmes, the political affairs counselor at the U.S. embassy in Kiev, Ukraine, will testify Thursday in the impeachment probe into President Donald Trump.

Two top national security aides who listened to President Donald Trump’s call with Ukraine are preparing to testify in the impeachment hearings, launching a week of back-to-back sessions as Americans hear from those closest to the White House.