WASHINGTON (Reuters) - President Donald Trump’s oldest son, Donald Trump Jr., will testify privately to the Senate Judiciary Committee on Thursday as it investigates allegations of Russian interference in the 2016 U.S. election.

FILE PHOTO: Donald Trump Jr. speaks at the 2016 Republican National Convention in Cleveland, Ohio U.S. July 19, 2016. REUTERS/Mario Anzuoni/File photo

Trump Jr. had been invited to testify in public in a hearing in July, but reached an agreement to speak privately with committee staff.

“We look forward to a professional and productive meeting and appreciate the opportunity to assist the committee,” Alan Futerfas, a lawyer for Trump Jr., said in a statement on Wednesday.

Russia has loomed large over the first seven months of the Trump presidency. U.S. intelligence agencies have concluded that Russia worked to tilt last year’s presidential election in favor of Trump, the Republican candidate.

Special counsel Robert Mueller is leading an investigation examining potential collusion with Russia by the Trump campaign. Several congressional committees are also looking into the matter, with the Senate and House intelligence committees conducting the main congressional investigations.

Trump Jr. is expected to testify before Senate Intelligence sometime later this year.

“I’m not interesting in talking to principal witnesses until I’ve talked to everybody else that was in the room,” Senator Richard Burr, the panel’s Republican chairman, told reporters.

Congressional investigators have focused on a June 9, 2016, meeting at Trump Tower whose attendees included a Russian lawyer and lobbyist and Trump associates including his son, and what came out of it, if anything, in terms of the relationship between Russians and the Trump campaign or Trump business interests.

Separately, Susan Rice, who was national security adviser for former President Barack Obama, testified on Tuesday before the House Intelligence Committee for about four hours.

Erin Pelton, a spokeswoman for Rice, said she had met voluntarily with the committee as part of its investigation. “Ambassador Rice remains fully supportive of bipartisan efforts to determine the extent and scope of Russia’s outrageous efforts to interfere in the 2016 election,” she said in a statement.

Rice had been subpoenaed by the committee as it looked into Republican concerns about whether anyone from the administration of Obama, a Democrat, had asked to “unmask” names of Trump campaign advisers picked up in top-secret foreign communications intercepts.

Several U.S. officials have told Reuters that all such requests by Obama administration officials were properly scrutinized and appropriate.

Moscow has denied any meddling. Trump denies collusion by his campaign.