WASHINGTON — Donald Trump Jr. will testify behind closed doors Thursday before the Senate Judiciary Committee about a 2016 meeting he attended with a Kremlin-linked lawyer offering dirt on Democratic presidential candidate Hillary Clinton.

His testimony, confirmed Wednesday by congressional aides who were not authorized to speak publicly, is part of the panel's investigation of the 2016 presidential race and possible collusion between the Trump campaign and Russian officials.

The president's son attended a June 2016 meeting with Russian lawyer Natalia Veselnitskaya at Trump Tower along with Paul Manafort, who was Trump's campaign manager at the time, and Jared Kushner, who is Trump's son-in-law.

Trump Jr. released emails in July of this year that showed he actively sought damaging information about Clinton from the meeting even after he was told it would come from the Russian government. Trump Jr. has described the meeting as a waste of time, saying it did not yield any useful information for his father's campaign.

The Judiciary Committee and the Senate and House Intelligence committees are all conducting Russia probes. Special counsel Robert Mueller is leading a separate investigation.

Senate Judiciary Committee staff and some senators will question Trump Jr. in Thursday's closed session.

"In late July, Donald Trump Jr. agreed to provide the Judiciary Committee with documents and a transcribed interview prior to a public hearing," said a joint Aug. 29 statement from Judiciary Chairman Chuck Grassley, R-Iowa, and the panel's senior Democrat, Sen. Dianne Feinstein of California.

Thursday's hearing is the next step in that agreement. The committee still plans to call Trump Jr. to testify publicly.

Read more:

Top Democrat fears House Intelligence Committee may split on Russia probe findings

Analysis: Donald Trump Jr.’s Russia emails, smoke meets fire

Trump Jr.'s Russian meeting: An unusual way of conducting political research

On Wednesday, the House Intelligence Committee met in closed session with Susan Rice, who served as national security adviser to former president Barack Obama.

Rice was reportedly questioned about the "unmasking" of Trump campaign aides by Obama-era intelligence officials as part of their investigation into Russia's interference in the 2016 election. The term "unmasking" refers to revealing the identities of people who were not the subject of an investigation but were swept up in a larger surveillance effort.

The issue has been raised by the White House and by Trump supporters, including House Intelligence Chairman Devin Nunes, R-Calif., to suggest that the Russia investigation has been politically motivated. But Democrats have largely dismissed it as an attempt to divert attention from possible collusion by the Trump campaign and Russians.