Update (May 14, 2019, 6:25 p.m. ET): This piece has been updated throughout to reflect the news that Donald Trump Jr. has agreed to interview by the Senate Intelligence Committee in June.

The Senate Intelligence Committee, led by Republican Sen. Richard Burr, has subpoenaed Donald Trump Jr. to appear and testify. On Tuesday, NBC News confirmed that Don Jr. would agree to appear, but would only speak on five or six topics. This move signals different things to different people. Some say it squarely contradicts Sen. Mitch McConnell’s “case closed” pronouncement from earlier this month. Indeed, this seems to open a new chapter in the Trump campaign-Russia investigation begun by Robert Mueller. Others say that Burr’s real intentions are to assist the administration, given his prior support for Trump through thick and thin.

Perhaps the committee legitimately wants to understand how and why Trump family members and associates interacted with Russian contacts. But I strongly suspect the question of what Congress hopes to accomplish by subpoenaing Don Jr. will ultimately go unanswered, especially the president's son is able to successfully limit the scope of his testimony.

If his track record is any indication, Don Jr. likely will attempt to use every trick at his disposal to keep from testifying.

The GOP-controlled Senate clearly does not believe that Attorney General William Barr’s redacted version of the Mueller report is sufficient. On May 9, President Trump told reporters that Don Jr. had told everyone who would listen about the Trump Tower rendezvous with the Russians on June 9, 2016, which the president called “a nothing meeting.” A quick fact check proves otherwise. The Mueller team was able to interview six of the eight participants in the infamous Trump Tower meeting. However, Mueller states that Don Jr. “declined to be voluntarily interviewed by this Office.”

Tellingly, the next sentence of the Mueller report is redacted for “Grand Jury” reasons. If we could peek under those black bars, we would almost certainly see that Don Jr.’s lawyers told Mueller that the president’s son would invoke his Fifth Amendment right against self-incrimination if forced to come before a grand jury. Accordingly, Mueller did not go through with the hollow exercise.

It’s worth noting that Don Jr.’s refusal to speak with Mueller tracks with his father’s inclination to stonewall. As the Mueller report relates in his report: “We also sought a voluntary interview with the President. After more than a year of discussion, the President declined to be interviewed.” Once again, the next sentence is redacted, but almost certainly states that the president’s lawyers indicated that Trump would invoke his Fifth Amendment right against self-incrimination if served with a grand jury subpoena. The fact that both Don Jr. and his father refused to be interviewed by Mueller belies the president’s repeated claims that he was completely cooperative and transparent vis-à-vis the Mueller investigation.

Why the reluctance to testify? Recall that he testified before the Senate Judiciary Committee on Sept. 7, 2017, stating that he was only “peripherally aware” of his father’s efforts to build a Trump Tower in Moscow. In televised interviews, he went even further, saying: “We didn’t know anything about it [the Trump Tower-Moscow deal]. Ultimately, it was Michael Cohen essentially trying to get a deal done.” This claim seems to take the “he was just a coffee boy” defense to new and absurd levels — suggesting that Michael Cohen was basically freelancing a multimillion-dollar Trump Tower deal without Trump family knowledge or involvement. Color me skeptical.

Cohen told Congress that he briefed Trump family members, including Don Jr., many times regarding the nature and progress of the Trump Tower-Moscow deal. Further testimony from Don Jr. could strengthen an already straight-faced perjury case for what appears to be his misleading testimony in September 2017.

According to reporting by NBC News' Ken Dilanian, Burr’s committee also wants to explore communications between Don Jr. and his father regarding the Trump Tower meeting. This sure sounds like the Senate is exploring potential misconduct by the president as part of legitimate congressional oversight of the executive branch. Could it also signal the Senate is starting to grapple with the question of whether to impeach the president? Only time will tell.

Many questions remain about what Don Jr. will or will not talk about. The Senate can extinguish a witness’s Fifth Amendment right against self-incrimination by granting the witness immunity and compelling the witness to testify. The downside of immunity is it can make future prosecution of the immunized witness more challenging, as the prosecutors would have to prove that none of the information obtained as a result of the immunized testimony would be used as evidence at a criminal trial. This exact issue ultimately led to the reversal of Oliver North’s criminal convictions for his participation in the Iran-Contra affair. The appellate court decided in 1990 that witnesses in North’s trial may have been improperly tainted by North’s immunized testimony before Congress.

Notably, even if Congress had immunized Don Jr., he could still have refused to testify. His father recently announced that he would honor no congressional subpoenas, an arguably unconstitutional interference with Congress’s oversight responsibilities. So far Barr, Treasury Secretary Steven Mnuchin and former White House counsel Don McGahn have all thumbed their noses at congressional attempts to investigate potential administration wrongdoing. And if Don Jr.'s legal team has anything to say about it, it's unlikely we're going to be hearing the whole truth and nothing but the truth from Don Jr. anytime soon.