Donald Trump Jr. is appearing Thursday before the Senate Judiciary Committee in a closed meeting. Congressional investigators are looking into whether the Trump campaign conspired with the Russian government during the 2016 election.

The president's oldest son became a key figure in the investigation after he agreed to a June 2016 meeting with a Kremlin-linked lawyer, along with Paul Manafort and Jared Kushner, after being promised dirt on Hillary Clinton.

Democrats on the committee want to know what happened in that meeting. They will also question Trump Jr. on his father's decision to fire FBI director James Comey, reports CBS News' Jeff Pegues.

On Wednesday, Facebook revealed that an internal investigation had determined the company had made approximately $100,000 selling 3,000 ads on the platform as a political influence operation by a Russian-linked "troll farm."

Facebook has reportedly given that data to special counsel Robert Mueller who is leading the Russia probe.

At the heart of the investigation is whether the Trump campaign was coordinating with Russian operatives. If the meeting isn't productive, Democrats have threatened to issue a subpoena because they really want him to testify in public.