Could the 39-year-old real estate scion be charged with treason? This is what legal experts say about what is known so far of the June 2016 meeting in New York between Don Jr and Natalia Veselnitskaya.
What law might he have broken?
The most incriminating revelation, say experts who spoke to AFP, are the emails that Donald Trump Jr dumped on his Twitter account: namely that he would "love" to hear what the Russian had to say about Hillary Clinton.
"If it's what you say I love it," he wrote back within minutes of being told the lawyer had information that would "incriminate" Clinton as "part of Russia and its government's support for Mr Trump."
Brandon Garrett, law professor at the University of Virginia, says since Watergate, federal law has forbidden any direct or indirect foreign contribution or promise of such to a US election.
The statute outlaws any donation "or other thing of value," which Garrett said could potentially include information.
Another worry for Donald Trump Jr is that conspiracy law is sweeping.
"One does not have to have committed a crime, the attempt is enough," said Garrett. "They could still be found guilty of conspiring," he warned. In other words, the sole fact of going into a meeting to fish for information could be enough to establish intent.
But Daniel Tokaji, an electoral law expert at Ohio State University, sees "very serious free speech" concerns and "troubling implications" in interpreting information as a "contribution."
He warned it "could prohibit a campaign from gathering information about a rival campaign."
Susan Klein, law professor at the University of Texas and a Democrat, doesn't believe Trump Jr could be prosecuted just for talking either -- "except perhaps if they paid for it or exchanged it for something."
There has been no suggestion, at least so far, that happened.
The Federal Election Commission (FEC) logically pursues election law violations, says Tokaji.
But split three-three between Democrats and Republicans, "it's very unlikely that they would take action," he added.
In theory the US attorney's office in Manhattan, where the meeting took place, could also open a case.
More likely is that former FBI director Robert Mueller, the special prosecutor charged with investigating possible links between the Trump campaign and Russia, will determine if there's a case, says Tokaji.
Whether the electoral law has been broken or not, he believes it is still worth investigating.
"It is quite troubling... that someone possibly acting at the behest of a foreign power is colluding with people at the very top (of a presidential campaign)," Tokaji said.
Did he commit treason?
Democratic Party lawmakers deployed the T word on Tuesday, but the experts who spoke to AFP doubted that would stick.
"Simply having a contact with a foreign country, even a foreign adversary -- we are not at war -- does not make it espionage or treason," says Joshua Dressler, law professor at Ohio State University.
"To bring it within treason, you'd have to (show that) they intended to harm their own country... That's an extreme argument to make," he said. "It would probably have to be something less extreme."