Then-National Security Adviser Michael Flynn. (Carolyn Kaster/AP)

Amid news that President Trump's former national security adviser Michael Flynn pleaded guilty Friday to lying to the FBI about his conversations with Russians, some legal experts are raising the question of whether Flynn just implicitly admitted to another crime: violating the Logan Act.

The Logan Act is a centuries-old law aimed at keeping private citizens out of foreign affairs. Flynn was a private citizen in December of 2016, and in Friday's guilty plea, he basically admitted he urged the Russian ambassador not to retaliate after President Obama announced sanctions punishing Russia for meddling in the presidential election. Authorities indicated he was acting at the urging of other senior Trump transition officials, suggesting they could have violated it too.

That sure sounds like it's possible Flynn or other Trump officials was trying to meddle in foreign affairs before he was a representative for the government, said Stephen Vladeck, a law professor at the University of Texas. It's possible the special counsel could whack Flynn or other campaign officials with this charge to get them to cooperate in the investigation.

Michael Flynn pleaded guilty on Dec. 1 to lying to the FBI about his contact with Russian officials. Court records indicate he was acting in consultation with senior Trump transition officials. (Jenny Starrs/The Washington Post)

But, Vladeck and other legal experts warn that it's not so easy. If Flynn or other campaign officials did get convicted with the Logan Act, it would be a first. The Logan Act has been around since 1799, and no one has ever been convicted of violating it. Plus, Flynn could have legal protections as a member of the transition team for the incoming president at the time.

Let's get a better understanding of this obscure law and how it could affect the Russia investigation.

Here's what it says in layman's terms: A citizen of the United States should not conduct U.S. foreign affairs without the permission of the government of the United States.

Here's the full legalese:

Any citizen of the United States, wherever he may be, who, without authority of the United States, directly or indirectly commences or carries on any correspondence or intercourse with any foreign government or any officer or agent thereof, with intent to influence the measures or conduct of any foreign government or of any officer or agent thereof, in relation to any disputes or controversies with the United States, or to defeat the measures of the United States, shall be fined under this title or imprisoned not more than three years, or both.

This section shall not abridge the right of a citizen to apply, himself or his agent, to any foreign government or the agents thereof for redress of any injury which he may have sustained from such government or any of its agents or subjects.

Here's its origin: It was named after George Logan, a state legislator from Pennsylvania who, in 1798, traveled to France to try to negotiate peace between the United States and France. (The two countries started fighting about whether the United States owed France its debts in the midst of the French Revolution. It was called the Quasi-War, and there's a whole Wikipedia entry on it here.)

Apparently the ruling Federalist Party, which controlled Washington at that time, was not happy about Logan trying to end said Quasi-War. When Logan returned home, he found the party had passed a law under his name forbidding any private citizen from conducting foreign affairs without the expressed permission of the government.

Interesting postscript: Despite having a federal law made specifically to rebuke him, Logan went on to serve in the U.S. Senate for six years.

Has anyone ever violated the Logan Act?: Plenty of people have been accused of violating it, but not a single person in the law's 200-plus-year history has gone to jail for it.

That's partly because the law is vague about which branch of government can give permission for someone to conduct foreign affairs, noted Eugene Volokh with The Volokh Conspiracy blog at The Post in 2015.

It could be the executive branch, it could be Congress. It could even be the president telling just one member of Congress to go do something without telling the others. It seems like are so many loopholes and uncertainties, it's hard for prosecutors to peg down how someone violated this law — and even harder for courts to make that determination.

But the law is pretty clear that you need to have permission from the government to do something, and it doesn't look like Flynn had permission to talk to Russians about sanctions while the Obama administration was still in power.

Still, Vladeck points out that Flynn could have a legal case to make if he were ever charged with this: He was acting on behalf of a president elect.

"It sure looks like this is  Logan Act violation,” he said. But, he continued, " ... it is not settled that transition officials are not acting with the authority of the United States."

Perhaps, he said, the use of the Logan Act would be cited in negotiations by the special counsel to put pressure on Flynn. Even if conviction would be difficult, no one wants to be charged with a violation. Perhaps, he said, Mueller would make use of the act in negotiations with Flynn “to call his bluff.”

It's mostly been used as a political weapon: To list just a few recent examples of instances when opponents have pointed to the measure:

  • In 2007, when House Speaker Nancy Pelosi (D-Calif.) went to Syria to negotiate with President Bashar al-Assad.
  • In 2015, when House Speaker John A. Boehner invited Israeli Prime Minister Benjamin Netanyahu to address Congress without President Obama's permission.
  • In 2015, when 47 Republican senators, led by Sen. Tom Cotton (R-Ark.), wrote a letter to Iranian leaders warning them not to agree to a nuclear deal the United States was negotiating.
  • And in 2016, when Republican presidential nominee Donald Trump stared straight into the cameras at a news conference and said: “Russia, if you’re listening, I hope you’re able to find the 30,000 emails that are missing.” The Senate's top Democrat at the time, Harry M. Reid (D-Nev.), said Trump “can't control his brain” is “being dumb” and accused him of violating the Logan Act.

Tom Hamburger contributed to this report.