WASHINGTON (AP) — Donald Trump Jr.'s scheduled visit to Capitol Hill on Thursday marks a new phase in the Senate investigation of Moscow's meddling in the 2016 election and a meeting that the president's eldest son had with Russians during the campaign.

Staff from the Senate Judiciary Committee, one of three congressional committees conducting investigations, planned to privately interview the younger Trump.

Emails he released in July that detailed preparations for the June meeting show he expected to receive damaging information about his father's opponent, Democrat Hillary Clinton, as part of what was described to Trump Jr. as a Russian government effort to aid the GOP nominee.

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

Senate aides also could pursue other possible connections that the president's family had with Russia.

Trump Jr. agreed to the interview after the committee chairman, Sen. Chuck Grassley, R-Iowa, subpoenaed him and Manafort. The committee withdrew the subpoenas after the two agreed to be interviewed privately by staff. Grassley said they both would eventually be questioned by senators in a public hearing.

Senators on the committee could attend the interview, but according to tradition they were not permitted to ask questions.

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 thought staff should be allowed to conduct the interview without interference.

She said staff interviews were intended to prepare their bosses for the public hearing, and "senators put a dent in it."

Feinstein said she and Grassley agreed to subpoena Trump Jr. and Manafort if they don't agree to attend a to-be-scheduled public hearing.

Trump Jr. also was expected to appear before the Senate Intelligence Committee at some point.

Sen. Mark Warner of Virginia, the top Democrat on the committee, said the panel wants to speak with others who attended the June meeting before interviewing 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 that committee in July. Kushner has met with that staff, as well as members of the House Intelligence Committee.

That House committee has tried to talk to Trump Jr., but Rep. Eric Swallwell, D-Calif., said negotiations are underway and a date hasn't been set.