Who is Keith Schiller, the latest White House aide to be questioned in Russia probe?

— -- Congressional investigators want to interview Keith Schiller, President Donald Trump's longtime bodyguard turned White House aide, as part of their probe into Russian interference in the 2016 presidential election, sources familiar with the investigation told ABC News.

Schiller, now the White House director of Oval Office operations, has been by the president’s side for nearly two decades. Schiller got his start with Trump during his days as a real estate mogul back in the 1990s.

If not often heard, Schiller is often seen by the president’s side -- a visible and regular presence and trusted insider.

His start

Long before arriving at the White House, and before he worked for Trump, Schiller was a New York City police officer.

And it was then, as a detective for the NYPD, that Schiller first encountered Trump at the Manhattan District Attorney’s office in the late 1990s.

Schiller would go on to join the Trump organization as a part-time bodyguard and would eventually rise through the ranks to the role of head of security at the organization.

Campaign

And when Trump transitioned from media mogul to presidential candidate, Schiller became a visible presence on the campaign trail.

He was swept up in headlines when he was caught in the middle of a heated press conference confrontation between Trump and Univision reporter Jorge Ramos.

At Trump's urging, Schiller stepped in and physically escorted Ramos out of the room after the reporter repeatedly questioned Trump about his immigration plan.

Schiller also stole headlines for punching a protester outside of Trump Tower in another physical altercation caught on camera during the campaign.

White House

Since coming to the White House, Schiller has assumed an elevated role as the director of Oval Office operations and has a desk located just outside the Oval Office.

He is regularly seen by the president’s side when he goes out in public and was part of a limited delegation that was invited to join the president in meeting Pope Francis at the Vatican.

The president sent Schiller along with his son-in-law and adviser Jared Kushner when he visited Iraq in April along national security adviser Lt. Gen. H.R. McMaster.

And when the president made the decision to fire FBI Director James Comey, it was Schiller who was tasked with hand-delivering the letter to FBI headquarters.