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

Schiller, the former head of security for the Trump Organization who now serves as the White House director of Oval Office operations, is one of several Trump associates on the House Intelligence Committee's witness list in its investigation into Russian interference.

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The committee's focus on Schiller and other Trump campaign officials and associates marks a new phase in the investigation, which is examining how Russia attempted to influence the election, the Obama administration's response and allegations of collusion between Trump associates and Russian officials.

It's the latest indication that the government's multiple investigations are touching Trump's inner circle. In late July, longtime Trump associate Roger Stone is expected to appear before congressional investigators for a closed-door interview. The growing list of other Trump associates the committee has said they want to meet includes former national security adviser Michael Flynn, Trump son-in-law and senior adviser Jared Kushner and former Trump campaign chairman Paul Manafort.

Kushner, Stone, Flynn and Manafort have all volunteered to cooperate with the investigations and have denied any wrongdoing.

Committee aides declined to comment about the addition of Schiller to its list of witnesses for ABC News. He did not respond to repeated requests for comment about the investigation.

One White House official was unsurprised to learn that Schiller has been contacted. As the investigations have expanded, several White House aides expressed privately to ABC News that they are expecting to hear from Congress or the special counsel.

A former New York police officer, Schiller has been at Trump's side for nearly 20 years and is one of his closest advisers and aides — playing the role of a body man, confidant and gatekeeper.

When Trump made the decision to fire FBI Director James Comey, it was Schiller who hand-delivered the letter of termination from the president to FBI headquarters.

Schiller accompanied Kushner on an April visit to Iraq with Marine Corps Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff.

Congressional investigators have interviewed dozens of witnesses as part of their probe, which continues as special counsel Robert Mueller leads a separate wide-ranging investigation at the Justice Department into Russian interference and whether there are any ties to the Trump campaign.

ABC News' John Santucci contributed to this report

Editor's Note: A previous version of this story incorrectly stated Schiller visited Iraq with Jared Kushner and national security adviser Lt. Gen. H.R. McMaster. Schiller visited Iraq in April with Jared Kushner and Gen. Joseph Dunford, the chairman of the Joint Chiefs of Staff.