WASHINGTON (AP) — President Donald Trump’s oldest son is scheduled to make his first appearance on Capitol Hill Thursday as part of a Senate investigation into Russian meddling in the presidential election and a meeting he had with Russians during his father’s campaign last year.

Donald Trump Jr. will be interviewed by staff on the Senate Judiciary Committee, which is one of three congressional committees investigating the meddling and possible Russian links to his Trump’s 2016 presidential campaign.

Trump Jr. released a series of emails in July that detailed preparations for the June 2016 meeting. The emails show he took the meeting expecting that he would be receiving damaging information about Democrat Hillary Clinton as part of what was described to him as a Russian government effort to aid the Trump campaign.

Special counsel Robert Mueller and the House and Senate intelligence committees are also investigating the meeting, which was attended by Trump’s son-in-law, Jared Kushner, and then-campaign chairman, Paul Manafort. A grand jury has heard testimony about it.

Staffers are expected to focus on the 2016 meeting but could also probe any other possible connections Trump’s family had with Russia. Trump Jr. agreed to the interview in July after Senate Judiciary Committee Chairman Chuck Grassley, R-Iowa, subpoenaed him and Manafort. The committee withdrew the subpoenas after the two agreed to be interviewed privately by staff, and Grassley said they both would eventually be questioned by senators in a public hearing.

Senators on the Judiciary panel are allowed to attend Thursday’s staff interview, but according to tradition they aren’t allowed to ask questions. On Wednesday, Democratic senators were split as to whether they would go and watch the proceedings.

Sens. Richard Durbin, D-Ill., and Richard Blumenthal, D-Conn., said they would be there. Sen. Chris Coons, D-Del., was considering it. California Sen. Dianne Feinstein, the committee’s top Democrat, said she would not attend because she thinks staff should be allowed to conduct the interview without interference. She said the staff interviews are intended to prepare their bosses for the public hearing, and “senators put a dent in it.”

Feinstein said that she and Grassley have agreed that they will again subpoena Trump Jr. and Manafort if they don’t agree to attend a public hearing. That hearing hasn’t been scheduled.

Trump Jr. is also expected to appear before the Senate Intelligence Committee, which is conducting its own investigation. Sen. Mark Warner of Virginia, the top Democrat on the committee, said Wednesday that the panel wants to speak with others who attended the June 2016 meeting before they interview Trump Jr.

“We want to do this in a thorough way that gets the most information possible,” Warner said.

Manafort met privately with staff on the Senate intelligence panel in July, and Kushner met with Senate intelligence staff and members of the House Intelligence Committee.

The House intelligence panel has also sought to talk to Trump Jr., but Rep. Eric Swallwell, a Democrat on that committee, said Wednesday that the panel is still negotiating the interview and a date hasn’t yet been set.