WASHINGTON — Former special counsel Robert Mueller is likely to face questions about one of the lingering mysteries in his report when he testifies before Congress on Wednesday: Why was Donald Trump Jr. the only American attendee of the infamous Trump Tower meeting in June 2016 not questioned by investigators?

Mueller’s report, released in April, states only that President Donald Trump’s eldest son “declined to be voluntarily interviewed by the Office.” But the next two lines in the report are redacted because they contain grand jury information.

The likely reason Trump Jr. wasn’t forced to talk with investigators is that his lawyer told the special counsel’s office that if he were subpoenaed he would invoke his Fifth Amendment right not to answer questions in an investigation that might incriminate him, former Justice Department officials told NBC News.

One of those officials, Chuck Rosenberg, former head of the Drug Enforcement Administration and an ex-U.S. attorney, said there are just two reasons why Mueller’s team wouldn’t have spoken with Trump Jr.: if officials didn’t view him as an important witness, which is highly unlikely given his pivotal role in arranging the Trump Tower meeting, or because he made clear to the special counsel’s office that he would invoke his Fifth Amendment right.

"There’s zero chance you wouldn’t want to talk to him, which means there’s a very good chance he would’ve invoked his Fifth Amendment right,” Rosenberg, an NBC News contributor, said.

Trump Jr.’s lawyer Alan Futerfas declined to comment on whether he told the special counsel’s office that his client would not testify and would invoke the Fifth if subpoenaed.

Mueller is scheduled to testify before the House Judiciary and Intelligence committees on Wednesday and has previously said any testimony he delivers won’t go beyond what he’s already said publicly or written in his report. But lawmakers are expected to press him on a number of points and have shown a particular interest in the president’s son and the Trump Tower meeting.

House Intelligence Committee Chairman Adam Schiff, D-Calif., in an interview Saturday, pointed to the Trump Tower meeting and Trump Jr.’s email exchanges about setting it up as the type of evidence in the investigation he intends to focus on during questioning of Mueller.

"You have all of the elements of conspiracy in the Trump Tower meeting,” Schiff told NBC’s Kristen Welker at the Aspen Security Forum. “You have, literally in writing, the Russians offering dirt on Hillary Clinton as part of what is described as the Russian government's effort to help Donald Trump. And you have the Trump campaign, in writing, accepting the offer."

In June 2016, Trump Jr. enthusiastically welcomed a Russian request for a meeting with a “Russian government attorney” promising “official documents” that would be damaging to his father’s opponent in the election, Hillary Clinton. Trump Jr. responded to the emailed request by saying, “if it's what you say I love it especially later in the summer."

He was told that the information "would be very useful to your father” and that it was “part of Russia and its government’s support for Mr. Trump.”

Trump Jr. convened the meeting with the lawyer six days later at Trump Tower.

Schiff said that while he sees little value in disputing Mueller’s conclusion that there wasn’t enough evidence to prosecute any Trump associates on charges of conspiring with Russia to influence the election, "it will be very important” for his committee to walk through the evidence the special counsel gathered as part of the investigation into potential collusion between Trump associates and Russia "because it is damning whether [it] can be prosecuted or not.”

"And that will be our focus,” Schiff said.

When Trump was asked in a recent interview with NBC News’ Chuck Todd whether his son had been subpoenaed, he said: “I don’t know, I don’t think so. You would need to ask him.”

When details of the Trump Tower meeting became public, Trump was already in the White House and the subject of a federal investigation into whether his campaign had colluded with Russians seeking to boost his election chances in 2016.

The Trump Tower meeting was a key focus of Mueller’s investigation. And Trump Jr.’s role in arranging the meeting was a core part of that line of inquiry. So it was notable that he was the only Trump associate at the meeting who wasn’t questioned in the investigation.

The notion that Trump Jr. would choose to invoke his Fifth Amendment rights also is notable given his father’s view of such a move.

While the Supreme Court ruled in a 2001 case that asserting one’s Fifth Amendment rights is not admission of guilt, Trump took a different view during his presidential campaign. During a September 2016 rally in Council Bluffs, Iowa, he said: “The mob takes the fifth. If you're innocent, why are you taking the Fifth Amendment?”

At the time, Trump was railing against Clinton by tying her to a computer expert at the State Department who also worked for her family and had asserted his Constitutional right not to testify before congressional committees investigating an attack on a U.S. consulate in Libya. Two other computer experts also asserted the right before a separate House committee investigating the attack, which took place while Clinton was secretary of state.

While Mueller’s investigation — which found Russia interfered in the election with the goal of helping Trump’s election — has wrapped up, Congress continues to investigate the matter.

Last month, Trump Jr. met with the Senate Intelligence Committee behind closed doors for more than three hours after he was subpoenaed to appear.

At issue was his knowledge of the Trump Tower meeting and his family’s efforts to secure a lucrative construction project in Moscow during the 2016 campaign.

The only person besides Trump Jr. who attended the Trump Tower meeting and wasn’t interviewed in the investigation was from the Russian side, according to Mueller’s report.

Natalia Veselnitskaya, the Russian lawyer who initiated the meeting, was in Russia so Mueller’s team was unable to question her.

Paul Manafort, who was Trump campaign chairman at the time, and Jared Kushner, the president’s son-in-law, also attended the Trump Tower meeting.

Manafort spoke with Mueller’s team under a cooperation agreement he entered into in hopes of a lighter sentence on charges unrelated to Russia’s election interference, though that agreement fell apart when prosecutors determined some of his testimony wasn’t truthful. Kushner voluntarily spoke with investigators multiple times.

CORRECTION: (July 23, 2019, 8:15 a.m. ET) An earlier version of this article misstated when the Trump Tower meeting took place. It was June 2016, not July 2016.