Donald Trump Jr. at a fundraiser for Faulkner University in Montgomery, Ala., in October. (Brynn Anderson/AP)

This post has been updated with news of Trump Jr.'s testimony. 

Donald Trump Jr. can shed light on pretty much every aspect of the Trump-Russia collusion investigation. He was a leading figure in the campaign, someone obviously very close to the candidate. We also know he had conversations with Russians and Russian-connected organizations during the campaign.

That's why the testimony of the president's son to Congress Wednesday was so crucial. It came as the special counsel investigation in particular seems to be heating up against top Trump campaign officials and getting them to flip sides and share what they know.

Trump Jr. testified to the House Intelligence Committee behind closed doors. He talked to Senate Intelligence Committee staffers and lawmakers in September. That testimony was also private, but Trump Jr.'s opening statement leaked to the media, and his explanations raised more questions than answers.

Here are three main unanswered questions lawmakers are likely to ask Trump Jr. about whether the Trump campaign colluded with Russia:

1. Did President Trump know about the meeting Trump Jr. held with Russians?

Details are slowly coming out about Donald Trump Jr.'s meeting with a Russian lawyer during his father's presidential campaign in June 2016, including a newly disclosed email from the lawyer to a music publicist who arranged the meeting. (Elyse Samuels,Jenny Starrs/The Washington Post)

Trump Jr. has never explicitly said whether the president knew about a June campaign meeting he held at Trump Tower with a Kremlin-connected lawyer. Trump Jr. took the meeting, his own emails show, because he was promised “dirt” on Hillary Clinton.

What his father knew about this meeting and when is important, given that legal experts have said that the meeting met the legal definition of collusion. But collusion isn't a legal term (conspiracy with a foreign government is), so he could be leaving himself and his father some plausible deniability.

“If his father knew about the meeting in advance and either condoned it or encouraged it, that is something [special counsel Robert S. Mueller III] would be very interested in,” said Jeffrey Jacobovitz, a white-collar criminal defense lawyer who has defended Clinton officials and is now at Arnall Golden Gregory LLP.

Jacobovitz noted that this question of what Trump knew and when is also coming at a time when at least one of Trump's lawyers is shifting the president's defense from: He didn't do anything wrong to he can't be charged for doing anything wrong. That suggests to Jacobovitz that the Trump team is worried about what investigators know, especially now that former national security adviser Michael Flynn has switched sides.

2. Why did Trump Jr. originally misstate what the Russia meeting was about, and why did the president not want him to reveal it?

President Trump personally dictated a statement that was issued after revelations that Donald Trump Jr. met with a Russian lawyer during the 2016 election. The Washington Post's Philip Rucker and Carol D. Leonnig explain. (Bastien Inzaurralde/The Washington Post)

This question gets to a pattern legal experts have noticed among key members of the Trump campaign and administration: They continually forget, misstate, mislead or even — in the case of Flynn — lie about their relations with Russia.

After the New York Times reported on the existence of the Trump Tower/Russia meeting, Trump Jr. first claimed it was about Russian adoptions. But his explanation for the meeting changed three times. Ultimately he settled on: He took the meeting to receive “very high level and sensitive information” that was “part of Russia and its government’s support for Mr. Trump.”

The Washington Post reports that Trump Jr. only issued the “adoptions” explanation to the New York Times after his own father dictated that he say the meeting was about adoptions.

On Wednesday, Trump Jr. refused to tell lawmakers about what he and the president talked about regarding how to respond to news of the meeting, said Rep. Adam Schiff (D-Calif.), the top-ranking Democrat on the House's intelligence committee.

Rep. Adam B. Schiff (D-Calif.), the ranking Democrat on the House Intelligence Committee, said Donald Trump Jr. did not answer some questions during a closed hearing on Dec. 6. (The Washington Post)

Whatever the president knew and when, add Trump Jr.'s changing explanation of Russia to the list.

“Why has everybody lied about it, forgotten about it, distorted what was happening and having to change their story?” said Asha Rangappa, an associate dean at Yale Law and a former FBI special agent.

3. What happened after the meeting?

In some ways, what happened afterward is more important than the fact the meeting took place, because it could provide evidence (or dispute allegations) that the Trump campaign actively worked with Russia. Did anyone brief Trump afterward? Were any documents or money exchanged? Did the Trump campaign get dirt on Clinton?

What happened after the meeting is especially pressing given that NBC News has reported that the Russian lawyer in the meeting, Natalia Veselnitskaya, says Trump Jr. asked her for evidence of any illegal Russian donations to the Hillary Clinton foundation.

If the Russians and the Trump campaign followed up on that and worked together, that could be a crime, Rangappa said.

Related to this, Trump Jr. will also have to explain why he exchanged private Twitter messages with WikiLeaks a month before the election. The head of the CIA has described WikiLeaks, which hacked into Democratic emails during the campaign, as a “non-state hostile intelligence service often abetted by state actors like Russia.”

Trump Jr. has kinda sorta denied anything else took place after the meeting, but he's left open the possibility: “I have no recollection of any documents being offered or left for us,” he told Senate investigators in September.

Congressional investigators and Mueller's parallel independent investigation will be the judge of that.