With his opening moves, Robert Mueller sent the message that his investigation into Russian interference in the 2016 election is serious, wide-ranging and far from over.
Now, as the special counsel continues his probe, his goal will be to uncover whether any more crimes were committed that can be prosecuted in court.
Perhaps surprisingly, the crime of "collusion" will not be one of them. In U.S. federal law, there is no crime that goes by that name, except in anti-trust cases.
Instead, Mr. Mueller and his team are searching for evidence of conspiracies to commit specific crimes, which could fall into several baskets: breaking election laws, violating laws against computer hacking or obstructing justice.
Mr. Mueller's first conviction, announced on Monday morning, was in the third category: George Papadopoulos, a former foreign-policy adviser to the Trump campaign, pleaded guilty to lying to federal agents about the extent and timing of his contacts with Russians.
On Tuesday, Mr. Trump lashed out at Mr. Papadopoulos on Twitter, calling him a "young, low level volunteer" who "has already proven to be a liar." (In March of last year, Mr. Trump had touted Mr. Papadopoulos as an asset to his team and an "excellent guy").
In April, 2016, Mr. Papadopoulos was told by a contact with ties to the Russian government that Moscow possessed "dirt" on then-Democratic candidate Hillary Clinton, including "thousands of e-mails." In the following weeks, he attempted to set up meetings between campaign staff and Russian officials, including trying to arrange a visit by Mr. Trump to Moscow.
Mr. Papadopoulos's eagerness to open channels with Moscow was echoed in a meeting that took place in June, 2016, between Donald Trump, Jr., Mr. Trump's eldest son, and a Russian lawyer at Trump Tower in New York. The younger Mr. Trump arranged the meeting after being promised incriminating information on Ms. Clinton. "If it's what you say I love it," wrote Mr. Trump, Jr., in response to the proposal.
While the behaviour of Mr. Papadopoulos and Mr. Trump Jr. might match the common-sense understanding of collusion – which is defined as a "secret agreement or co-operation especially for an illegal or deceitful purpose" – it is not clear yet whether any crime was committed.
Collusion "has no legal meaning whatsoever," said Renato Mariotti, a former federal prosecutor in Chicago. But, he added, "conspiracy" is a similar concept – an agreement between two people to commit a particular crime, an agreement which is itself illegal.
The hacking of the e-mail servers of the Democratic National Committee, for instance, was a federal crime. If anyone connected to the Trump campaign conspired with or aided those who carried out that illegal act, they would face criminal jeopardy, said Mr. Mariotti. It is also a federal crime to receive stolen goods worth at least $5,000 (U.S.), he noted, something that could apply to that cache of e-mails.
Meanwhile, the behaviour of Mr. Papadopoulos and Mr. Trump Jr. raises the possibility of a different kind of criminal prosecution, experts say.
Under federal election law, it is illegal to knowingly solicit or accept any contribution or any "thing of value" from a foreign national in connection with an election.
The prohibition on foreign contributions is the "most salient, direct crime" potentially under investigation, said Paul Rosenzweig, a former senior official in the Department of Homeland Security. "Nobody has ever asked whether giving 'dirt' or 'oppo' research would constitute the type of prohibited assistance that is like giving money. [The answer] is likely yes."
Ciara Torres-Spelliscy, an expert on election law at Stetson University, said the prohibition on foreign contributions is rarely prosecuted "at least in part because most people who work on presidential elections are cognizant of the fact that this is patently illegal."
Prof. Torres-Spelliscy pointed to a now-forgotten episode in 2016 in which the Trump campaign solicited donations via e-mail from lawmakers in Britain, Australia and Iceland. "From the outside, it looks like there are potentially multiple violations on the prohibition on foreign money and foreign donations of value during the 2016 presidential campaign," she said.
Another area of focus for Mr. Mueller's investigation is whether any of Mr. Trump's advisers – or even, potentially, the President himself – acted unlawfully to thwart the probe. Prior presidential scandals have shown that efforts to obstruct investigations are often where the White House gets into perilous legal territory.
As Mr. Mueller weighs whether any criminal conduct occurred, he will have the ability to draw on a unique resource. One member of his team is Michael Dreeben, a Justice Department official who has argued more than 100 cases before the U.S. Supreme Court and is widely considered the foremost expert on criminal law in the country.
"Any steps they take will have been run through Michael Dreeben, and he will have said, 'We can defend these [moves] in court,'" Mr. Rosenzweig said. When Mr. Dreeben argues something, he added, "the Supreme Court listens."