Did Donald Trump Jr break the law?
During the 2016 campaign, the eldest son of Donald Trump met a woman presented to him as a "Russian government attorney" after being told she had damaging information about his father's presidential election rival Hillary Clinton. Did he commit a crime?
Donald Trump Jr was joined at his 9 June 2016 meeting with attorney Natalia Veselnitskaya by then-Trump campaign manager Paul Manafort and Jared Kushner.
In an interview with Fox's Sean Hannity on Tuesday night, Mr Trump Jr insisted Ms Veselnitskaya provided nothing of value.
But legal scholars say that whether or not the interaction was productive in Mr Trump Jr's mind is immaterial, and he could have committed a serious federal offence.
What laws could have been broken?
Many legal commentators say Mr Trump Jr's correspondence with Rob Goldstone, who set up the meeting, and the subsequent meeting with Ms Veselnitskaya possibly constitutes a violation of US campaign statute.
Specifically, Section 30121 of Title 52 of federal campaign law, which deals with "contributions and donations by foreign nationals".
"It is illegal for a foreign government or a foreign national to give something of value to a candidate or campaign," says Nathaniel Persily, an election law expert and professor at Stanford Law School.
"It is also illegal for a foreign national and a foreign government to spend money in connection with a federal election."
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The law prohibits foreign nationals from providing "a contribution or donation of money or other thing of value" to American federal, state and local election campaigns.
However, whether or not damaging information about Mrs Clinton, the former Democratic presidential candidate, could be considered a "thing of value" is a legal grey area, according to Richard Briffault, a professor at Columbia Law School, and will be key to any future legal consequences for Mr Trump Jr.
"Is providing information covered by this? That's not clear," says Professor Briffault. "I don't know if there's any specific rule or past case that includes information itself."
Could any other laws have been broken?
According to Richard Painter, former chief White House ethics lawyer for George W Bush, if Mr Trump Jr knew that the Russians were engaged in hacking and that the information he was seeking was obtained by these means, he could have violated the Computer Fraud and Abuse Act.
"It's like going into a store to buy goods you know are stolen," he says.
The 9 June 2016 meeting took place after the Russian hacking of Democratic National Committee emails, but before the hack was made public.
Additionally, if Mr Trump Jr made any misleading or false statements to federal investigators about his contact with Russian representatives, he could face felony criminal charges.
Mr Trump Jr has yet to testify under oath publicly on these matters. But it is possible that behind the scenes, he has given interviews to US intelligence agents or signed sworn statements in the course of the campaign and his father's presidency.
All of that is yet to be determined.
"It clearly cries out for further investigation," says Professor Briffault.
Is this treason?
Former Clinton running mate Senator Tim Kaine told CNN that the investigation is "moving into perjury, false statements, and even into potentially treason".
But Professor Briffault played that down.
"Treason strikes me as a little over the top," he says.
Richard Painter, the Bush administration ethics lawyer, has used the term in interviews, but said that because we are not at war with Russia, the treason statute likely would not apply.
"In situations where we don't have a declared war we use other statutes to prosecute conduct," he says.
Jonathan Turley, professor of public interest law at George Washington University, has strenuously pushed back on the idea this is treason, or a violation of the Logan Act, which prohibits US citizens from meddling in disputes between the American government and foreign nations.
"[The Logan Act] has never been used to convict a single US citizen and is widely viewed as facially unconstitutional," wrote Professor Turley in The Hill.
So what about 'collusion'?
There are many legal experts - Vox found 12 - who believe that the emails prove "collusion". However, collusion simply means that two parties were willing to work together in secret, and is not a prosecutable offense. Many lawyers and scholars are pleading with journalists to stop using what they say is a legally useless term.
"We're talking about 'collusion' as a kind of a catch-all for what are specific, potential crimes," says Professor Persily.
There are, however, laws against "conspiracy", defined by former federal prosecutor Andrew C McCarthy in the Washington Post as "an agreement to commit a concrete violation of law" between two or more parties.
Based on what is currently public, it is unclear whether or not a conspiracy charge would apply here.
"To rise to the level of conspiracy, there would need to be proof, for example, that a) violations of U.S. law were orchestrated by the Russian regime, and b) Trump campaign officials knew about them and were complicit in their commission," McCarthy wrote.
Whose job would it be to charge Trump Jr?
If Mr Trump Jr were ever to face criminal charges over his conduct, those would come from Robert Mueller, the former FBI director and special counsellor appointed to investigate the Trump campaign's potential ties with Russia.
"Whether or not Don Jr may have violated the law is something that special counsel Robert Mueller and his talented team will now investigate," writes Jennifer Taub, a professor at Vermont Law School, in an email to BBC.
But she cautions that it is much too early to make predictions on whether or not that will occur.
"This is just one thread that has been pulled. There is so much more to come as this unravels," she writes.
Whether or not President Trump himself will be affected rests on the outcome of the ongoing federal investigation.
Still to be determined is the famous question asked during the investigation into the Nixon-Watergate scandal: What did the president know, and when did he know it?