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The Senate Intelligence Committee has subpoenaed Donald Trump Jr. for him to return and testify again, and the committee is now at a standoff with President Donald Trump’s eldest son, according to sources familiar with the matter.

One option Trump Jr. is considering in response to the subpoena is to invoke his Fifth Amendment rights, and another is just to not appear at all, according to one source. The subpoena from the Republican-led Senate Intelligence panel is believed to be the first issued to one of Trump’s family members.

Discussions for Trump Jr.’s testimony began several weeks ago before special counsel Robert Mueller’s report was released, the sources say. Trump Jr.’s team resisted giving testimony, in part, because the findings of the Mueller report were still not known.

During the negotiations, the idea to use written questions and answers was floated, and at another time it was proposed that Trump Jr. sit for an untranscribed interview, according to one source.

The subpoena was issued more than two weeks ago, according to a source familiar with the matter, and it compelled Trump Jr. to testify before the committee, the source said.

The subpoena for Trump Jr.’s testimony marks an escalation of the Senate Intelligence Committee’s probe into Russian election interreference. The panel’s investigation, led by GOP Sen. Richard Burr of North Carolina, has been running for more than two years, and the committee has interviewed many of the same witnesses who spoke to Mueller’s team. The committee has recently begun re-interviewing witnesses, including Trump’s son-in-law, Jared Kushner, who appeared for a second time earlier this year.

A committee spokesperson said: “We do not discuss the details of witness engagements with the committee. Throughout the investigation, the committee has reserved the right to recall witnesses for additional testimony as needed, as every witness and witness counsel has been made aware.”

A source close to Trump Jr. said in a statement Wednesday that when Trump Jr. testified in 2017, there was an agreement “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.”

“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 source said.

Other Republicans also criticized Burr for his pursuit of Trump Jr.

“Apparently the Republican chair of the Senate Intel Committee didn’t get the memo from the Majority Leader that this case was closed…” tweeted Kentucky Republican Sen. Rand Paul on Wednesday, referencing a speech from Senate Majority Leader Mitch McConnell on Tuesday in which he declared “case closed” on the special counsel investigation.

Trump Jr.’s position on testifying again hardened after the Mueller report was released, according to one of the sources. The report stated that the special counsel’s team explored charging Trump Jr. and other campaign officials with campaign finance violations for the meeting they took with a Russian lawyer offering dirt on Hillary Clinton in 2016 but ultimately declined because they could not place a value on the materials offered and could not establish Trump Jr. knew he might be violating the law. Mueller’s team also did not establish that there was a conspiracy between Russians and members of the Trump campaign, which Trump allies have used to embolden their position that the investigation was a “witch hunt.”

Axios first reported the subpoena.

There are multiple issues that the Senate panel is likely interested in asking Trump Jr. about, as he’s one of the witnesses that the committee’s top Democrat, Sen. Mark Warner, has suggested should return to the committee for additional questioning.

Trump Jr. has already testified under oath in 2017 before both the House and Senate intelligence committees and the Senate Judiciary Committee. The Senate Judiciary Committee released the transcript publicly.

Trump Jr. declined to speak voluntarily to Mueller, according to the Mueller report, and the special counsel did not seek to subpoena him.

Trump Jr.’s testimony has been scrutinized for how he described his conversations in the lead-up to the June 2016 Trump Tower meeting, in which he said that he only talked to Kushner and former Trump campaign chairman Paul Manafort about the meeting where a Russian lawyer was offering dirt on the Hillary Clinton campaign.

“Was there anyone else?” Trump Jr. was asked.

“No, not to my recollection,” he responded.

But in the Mueller report, the special counsel wrote that Trump Jr. announced at a morning meeting of top campaign and Trump family members “he had a lead on negative information about the Clinton Foundation” in the days before the Trump Tower meeting. Rick Gates, Manafort’s former deputy who was charged and pleaded guilty in the special counsel’s investigation, told Mueller’s team about the discussion.

Trump Jr. could also face questions about the Trump Tower Moscow project that Trump’s former attorney Michael Cohen worked on in 2015 and 2016. Trump Jr. was the only one of Trump’s children who has testified before Congress about the project.

Mueller’s report says that Cohen testified he had discussed the project on multiple occasions with Trump Jr.

“Cohen also recalled briefing Donald Trump Jr. in the spring – a conversation that Cohen said was not ‘idle chit chat’ because Trump Tower Moscow was potentially a $1 billion deal,” the report says.

CNN’s Kaitlan Collins contributed to this report.