Fiona Hill, President Trump's former top Russia adviser, and David Holmes, the counselor for political affairs at the US Embassy in Ukraine, are testifying together publicly today before the House Intelligence Committee in the Democrat-led impeachment inquiry into Trump.

The hearing is scheduled to start at 9 a.m. ET.

Here's what we know about Hill and Holmes:

About Hill: A former national security official, Hill served in the Trump administration from April 2017 until July of this year. During her time with the National Security Council, she oversaw rocky Washington-Moscow ties, and her views sometimes seemed at odds with Trump's desire to improve relations with Russian President Vladimir Putin whom he has praised on multiple occasions.

In her previous deposition, Hill said US Ambassador to the European Union Gordon Sondland told Ukrainian officials in meetings on July 10 they would have to open an investigation to secure a meeting between Trump and Ukrainian President Volodymyr Zelensky.

About Holmes: Holmes is a career foreign service officer who arrived in Ukraine in 2017, according to a source who knows him and describes him as a "sharp guy." He joined the foreign service in 2002, according to the American Foreign Service Association, and has previously served in Kabul, New Delhi, Kosovo, Bogota, Moscow and Kosovo.

He told lawmakers during a closed-door deposition that he overheard a conversation between President Trump and Ambassador Gordon Sondland the day after Trump spoke with the Ukrainian president by phone in July, CNN previously reported. Holmes heard Trump ask Sondland on the call if the Ukrainians were going to "do the investigation," and Sondland responded, "He's gonna do it."