Flynn's contact with the Russian ambassador "seems to violate what the Logan Act intended to prevent," Zeldin said. He added that even though the Logan Act hasn't been used successfully "it doesn't mean that Mueller wouldn't consider using it to pressure defendants."
The Logan Act, which was passed in 1799, 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."
"I think it's a pressure point for other related offenses," said Stephen Vladeck, a professor at the University of Texas law school and a CNN contributor. "At the end of the day, there's no way in which any indictment is going to rise and fall on the Logan Act. The question is whether there's some broader conspiracy. It increasingly looks like a pretty wide-reaching cover up."
Lawyers say the conversations could also be used by Mueller's team as it looks for possible evidence of conspiracy, a quid pro quo between Trump campaign officials and Russians or pursues a case of obstruction of justice. In court filings, Mueller's team stated: "Flynn's false statement and omissions impeded and otherwise had a material impact on the FBI's ongoing investigation into the existence of any links or coordination between individuals associated with the campaign and Russia's efforts to interfere with the 2016 presidential election."
A senior White House official on Friday defended Flynn's conversations and claimed the Obama White House knew about the discussions. "None of the conversations were improper and were authorized by the Obama administration," the official said.
Court filings show that Flynn said he was directed by a "very senior member of the presidential transition team" to make contact with foreign governments, including Russia, on December 22, 2016, to "delay or defeat a UN resolution to condemn Israeli settlements."
A source familiar with the investigation said that Jared Kushner, Trump's son-in-law and senior adviser, is the "very senior" transition official mentioned in the documents. Kushner met with Mueller's investigators last month to discuss Flynn, CNN has reported.
The following day, the Russian ambassador told Flynn Russia would not vote against the resolution. The resolution passed.
An attorney for Kushner did not comment.
: "As to the U.N., things will be different after Jan. 20th."
As to the U.N., things will be different after Jan. 20th.— Donald J. Trump (@realDonaldTrump) December 23, 2016
: "The big loss yesterday for Israel in the United Nations will make it much harder to negotiate peace. Too bad, but we will get it done anyway!"
The big loss yesterday for Israel in the United Nations will make it much harder to negotiate peace.Too bad, but we will get it done anyway!— Donald J. Trump (@realDonaldTrump) December 24, 2016
Flynn also admitted to making false statements to the FBI about conversations he had with Kislyak beginning on December 28, 2016, about sanctions enacted by the Obama administration. In one call, Flynn discussed with "a senior official" on the Trump transition team who was with other transition officials at Trump's Mar-a-Lago resort that they not want Russia to escalate the situation in light of the sanctions.
The following day, Russian President Vladimir Putin said Russia would not retaliate against the US for the sanctions.
KT McFarland was the senior transition official at Mar-a-Lago who was described as discussing with Flynn what, if anything, to communicate to the Russian ambassador about US sanctions, according to sources familiar with the matter.
An attorney for McFarland declined comment.
CNN's Marshall Cohen and Jim