Congressional investigators to question longtime Trump aide

— -- A longtime aide to President Donald Trump who followed the businessman-turned-president from New York City to the White House is expected on Capitol Hill Tuesday to face questions from congressional Russia investigators.

Keith Schiller, who served in the West Wing as Trump’s director of Oval Office operations until September, will meet with the House Intelligence Committee behind closed doors as the panel investigates Russian election interference and allegations of collusion between the Russia and the Trump campaign.

ABC News first reported congressional investigators’ interest in Schiller earlier this year.

A former New York City police officer, Schiller has been a fixture at Trump’s side for decades, serving as a body man for the New York developer, and a point of contact for many people trying to get in touch with Trump.

He is expected to face questions from Democrats about Trump's 2013 trip to Moscow for the Miss Universe pageant, sources told ABC News. Schiller joined Trump on that trip, which is the focus of some of the salacious, unverified allegations in the controversial dossier produced by former British spy Christopher Steele.

Steele was hired by political research firm Fusion GPS to dig up dirt on Trump. The firm was retained by the Democratic National Committee and Hillary Clinton’s presidential campaign after initially working for the Washington Free Beacon, a conservative news organization, during the 2016 GOP presidential primary.

In May, Schiller hand-delivered Trump’s letter firing former FBI Director James Comey to FBI headquarters. Special counsel Robert Mueller is now investigating the circumstances around Comey’s firing.

He was also known on the campaign trail as Trump’s enforcer, physically removing Univision anchor Jorge Ramos from a 2015 press conference, and hitting a protester in the face outside another Trump Tower event.

Schiller did not respond to a request for comment on his appearance before congressional investigators.

ABC News' John Santucci contributed to this report.