The Senate Intelligence Committee reportedly subpoenaed Donald Trump Jr. after the president's eldest son backed out of two scheduled interviews regarding the panel's ongoing investigation into Russian election interference, The Associated Press reported Tuesday.

Committee Chairman Richard Burr (R-N.C.) told colleagues at a GOP caucus luncheon last Thursday that Trump Jr. voluntarily agreed to be interviewed twice but later withdrew, three people familiar with the remarks told the AP.

Trump Jr. and the committee had been in negotiations since December and interviews were scheduled for March and April, sources familiar with Burr's remarks told the news service.

Burr detailed the scheduled interviews for his Republican colleagues after he faced blistering criticism for issuing a subpoena for Trump Jr. - the first known congressional subpoena to one of the president's children.

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 appearances.

Senators on the Intelligence Committee want to go over Trump Jr.'s 2017 interview and ask further questions, the AP reported.

His testimony came under scrutiny after the president's former personal attorney Michael Cohen claimed in his February congressional testimony that Trump Jr. was far more involved in the Moscow Trump Tower project than initially thought and that he briefed both Trump Jr. and Ivanka Trump on the project about 10 times.

Cohen pleaded guilty last November to lying about discussions on the Moscow plans within the Trump Organization.

Senate Judiciary Committee Chairman Lindsey Graham (R-S.C.) said Monday that Trump Jr. should comply with the Senate Intelligence Committee's subpoena but refuse to answer any questions.

"You just show up and plead the Fifth and it's over with," Graham told reporters, referring to the amendment that protects citizens from self-incrimination, according to The Washington Post.

The Senate Intelligence Committee has been conducting its own investigation in parallel with special counsel Robert Mueller into Russian election interference in 2016, which concluded earlier this year.

Trump Jr. did not speak to the special counsel, and the Mueller report released last month detailed how Mueller's office did not find sufficient evidence to prove that any Trump campaign associates conspired with the Russian government to meddle in the 2016 race.