The Republican-led Senate Intelligence Committee has subpoenaed Donald Trump Jr. Don John Trump'Tiger King' star Joe Exotic requests pardon from Trump: 'Be my hero please' Zaid Jilani discusses Trump's move to cancel racial sensitivity training at federal agencies Trump International Hotel in Vancouver closes permanently MORE in connection with the panel’s Russia investigation, the first known congressional subpoena to one of President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s children.

Axios first reported the subpoena, and a source confirmed it to The Hill.

Trump Jr. already testified behind closed doors before the House and Senate Intelligence panels in December 2017 as part of their investigations into Moscow's interference in the 2016 election. Neither committee has released a transcript of his closed-door hearing.

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The president’s eldest son also testified to the Senate Judiciary Committee in September 2017, telling lawmakers that he was “peripherally aware” of plans to expand his father’s businesses into Russia, according to a transcript that was later released by the committee.

Trump Jr.’s testimony has fallen under scrutiny, particularly after the president’s former personal attorney Michael Cohen Michael Dean CohenJudge orders Eric Trump to comply with New York AG's subpoena before Election Day A huge deal for campaign disclosure: Trump's tax records for Biden's medical records Our Constitution is under attack by Attorney General William Barr MORE claimed in his February testimony to the House Oversight and Reform Committee that Trump Jr. was far more involved in the Moscow Trump Tower project and that he briefed both Trump Jr. and Ivanka Trump Ivana (Ivanka) Marie TrumpSpecial counsel investigating DeVos for potential Hatch Act violation: report Trump, Biden vie for Minnesota Trump luxury properties have charged US government .1M since inauguration: report MORE on the project about 10 times.

Cohen, who pleaded guilty last November to lying about discussions on the Moscow plans within the Trump Organization, delivered marathon closed-door testimony before the Senate Intelligence Committee a day before his House appearance.

A Senate Intelligence spokesperson would not confirm the subpoena, but noted in a statement to The Hill that the panel has “reserved the right to recall witnesses for additional testimony as needed.”

"We do not discuss the details of witness engagements with the Committee," the spokesperson said.

It was not immediately clear how Trump Jr. would respond to the subpoena. His attorney, Alan Futerfas, did not immediately return a request for more information.

A source close to Trump Jr. criticized Democrats for issuing the subpoena, noting that he had already appeared voluntarily before the Senate Intelligence Committee.

"Don is a private citizen, who has already been cleared by Mueller after a two-year investigation," the source said, referring to special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE.

"He has done 8-9 hours of testimony in front of Senate Intel already and 27 hours of testimony in front of various committees in total," the source said.

The source also took a shot at the panel's chairman, Sen. Richard Burr Richard Mauze BurrRep. Mark Walker says he's been contacted about Liberty University vacancy Overnight Defense: Trump rejects major cut to military health care | Senate report says Trump campaign's Russia contacts posed 'grave' threat Senate report describes closer ties between 2016 Trump campaign, Russia MORE (R-N.C.), by arguing that he should have stood up to ranking member Mark Warner Mark Robert WarnerDemocrats call for declassifying election threats after briefing by Trump officials It's time to upgrade benefits Intelligence chief says Congress will get some in-person election security briefings MORE (D-Va.) and not insisted on additional testimony from Trump Jr.

"When he originally agreed to testify in front of the Senate Intel Committee in 2017, there was an agreement between Don and the Committee that he would only have to come in and testify a single time as long as he was willing to stay for as long as they’d like, which Don did," the source said.

"Don continues to cooperate by producing documents and is willing to answer written questions, but no lawyer would ever agree to allow their client to participate in what is an obvious PR stunt from a so-called 'Republican' senator too cowardly to stand up to his boss Mark Warner and the rest of the resistance Democrats on the committee."

The Senate Intelligence panel has been conducting its investigation into Russia's election interference since January 2017, and Burr has signaled he expects the probe to wrap up in the coming months as the panel finishes interviewing additional witnesses.

Trump son-in-law and senior adviser Jared Kushner Jared Corey KushnerAbraham Accords: New hope for peace in Middle East Tenants in Kushner building file lawsuit alleging dangerous living conditions Trump hosts Israel, UAE, Bahrain for historic signing MORE was spotted returning to the committee for closed-door testimony in late March, reportedly in connection with the Russia investigation.

The Senate probe has run parallel to special counsel Robert Mueller’s Russia investigation, which concluded on March 22 without a recommendation of further indictments.

Mueller did not establish that members of the Trump campaign conspired with the Russian government and did not reach a conclusion on whether Trump obstructed justice.

Trump Jr., who did not speak with the special counsel, was featured in the report, including in a section detailing his involvement in a June 2016 meeting between the campaign and a Kremlin-backed lawyer at Trump Tower.

—Jonathan Easley contributed.