(CNN) Michael Flynn is out as President Donald Trump's national security adviser. But does he have bigger problems than just needing a new job?

At issue is the little-used Logan Act, which forbids private citizens from negotiating with foreign governments.

Some vocal critics have suggested Flynn could be prosecuted for violating the statute. But no one has previously been prosecuted under the law, and no one in the law enforcement community has suggested Flynn would be.

Here's a look at the law:

What is the Logan Act?

The Logan Act forbids private citizens "without authority of the United States" from negotiating with foreign governments with an "intent to influence" measures or conduct of that government regarding any "disputes or controversies."

Congress passed it in 1799 in response to the actions of George Logan, a Pennsylvania doctor who went to France as a private citizen and tried to negotiate with officials there, according to the Congressional Research Service

The legal bottom line here: Only the President and the President's authorized emissaries have the power to negotiate with other countries.

What does it have to do with Flynn?

As a member of Trump's transition team, Flynn spoke by phone on December 29 with Russian Ambassador Sergey Kislyak. The calls happened as President Barack Obama's administration was issuing sanctions against Russia for its alleged interference in the 2016 US election.

When news of Flynn's phone calls became public in January, administration officials, including Vice President Mike Pence, said the conversations focused on logistics, not sanctions.

"They did not discuss anything having to do with the United States' decision to expel diplomats or impose censure against Russia," Pence told CBS

But US investigators looked into the claim, and the Justice Department last month warned the Trump administration that Flynn had misled administration officials about his communications with Kislyak and could be vulnerable to blackmail by the Russians.

The White House stood behind Flynn -- until it didn't. The retired Army lieutenant general resigned Monday night, admitting he had "inadvertently briefed the vice president-elect and others with incomplete information" about the Kislyak phone calls.

Did Flynn break the law?

This case hits several key aspects of the Logan Act: 1. whether Flynn had "authority" to participate in the phone calls; 2. whether conversations related to a "dispute or controversy"; and 3. whether he had an "intent to influence" a foreign government.

Let's break those down:

There's no evidence Flynn, as an adviser at the time to President-elect Trump, had authority from Obama to speak with foreign officials. A US official confirms that Flynn and Kislyak did speak about sanctions. The exact context of Flynn's side of the conversation was not clear, even to the FBI and intelligence groups that reviewed the content, a US official told CNN.

Given that third point, it's unclear whether Flynn's conversations related to a "dispute or controversy," or whether he had an "intent to influence" a foreign government.

White House Press Secretary Sean Spicer on Tuesday said the White House counsel looked into the situation and found no "legal" issue with the phone calls.

"The question is not whether he did anything improper or illegal," Spicer said. "It was not an issue of law; it was an issue of trust."

Some FBI and Justice Department officials have told CNN the phone calls are not likely to lead to any criminal charges.

Several senators have called for an investigation into Flynn and the Trump administration more broadly to learn what was said in those phone calls, along with who knew about their content and when.

What about past Logan Act cases?

Here's the rub. The 218-year-old Logan Act has almost no track record, and no one has ever been prosecuted under it.

Only once, in 1803, was someone even indicted: a Kentucky farmer who wrote an article advocating for a separate Western nation allied with France. But the Louisiana Purchase later that year made the matter obsolete, the Congressional Research Service reported.

The Logan Act "is a relic of a bygone era," said Steve Vladeck, a CNN contributor and law professor at University of Texas School of Law.

Still, threats of prosecution pop up every few years:

President Ronald Reagan accused the Rev. Jesse Jackson of violating the act in 1984 when Jackson visited Moscow in an attempt to secure the release of a dissident.

Two years ago, 47 Republican senators signed an open letter to Iran's officials declaring their opposition to the Iran nuclear deal. Critics suggested that may have violated the Logan Act.

"It gets trotted out every time there's a political disagreement when someone who is not the president touches on foreign policy," Vladeck said. "It's the old chestnut that everyone quotes and no one understands."

So will Flynn face Logan Act charges?

The Logan Act likely won't be used here, and it may not even be enforceable at all anymore, Vladeck said.

The law essentially criminalizes speech, he said, and the Supreme Court looks very skeptically at laws that may infringe upon that right.

In addition, the law's mention of a person acting "without authority of the United States" could be an escape hatch, Vladeck has written

Flynn wasn't simply a private citizen; he was an adviser to the President-elect and soon would be given diplomatic powers. Legally speaking, his role may have granted him the "authority" to talk sanctions with Kislyak, Vladeck's interpretation suggests.

What about other laws?

While the Logan Act likely won't be an issue, Flynn could face other legal challenges related to the phone calls.

"The real question is whether Flynn made false comments to any investigators," Vladeck said. "But no, the Logan Act, I think, is full of sound and fury but signifying nothing."

Could other officials face charges?

It's too early to say.

A number of Democratic and Republican senators have called for an investigation into President Trump's connections with Russia and want Flynn to testify as part of that inquiry.

"What did (Trump) know about Flynn's dealings (with) Russians and when did he know it?" David Axelrod, a CNN senior political commentator and a former senior Obama adviser, asked on Twitter.