By Sarah N. Lynch

WASHINGTON (Reuters) - Former U.S. national security adviser Michael Flynn pleaded guilty on Friday to lying to the FBI about contacts with Russia and agreed to cooperate with prosecutors delving deeper into the actions of President Donald Trump's inner circle before he took office in January.

The dramatic turn of events also raised more questions about the actions of Trump's son-in-law, Jared Kushner. Flynn admitted to lying about asking the Russian ambassador to help delay a U.N. vote seen as damaging to Israel and Kushner, according to sources, was identified as the "very senior official" who told Flynn to contact Russia and other countries to try to influence their votes.

Flynn became the first member of Trump's administration to plead guilty to a crime uncovered by the wide-ranging special counsel investigation into Russia's alleged attempts to influence the 2016 election and potential collusion by Trump aides.

Flynn, a former senior member of Trump's campaign team, admitted in court in downtown Washington that he gave false statements to the Federal Bureau of Investigation in January about his contacts the previous month with Russia's then-ambassador, Sergei Kislyak.

“Guilty, your honor,” the retired army general said, when asked by U.S. District Judge Rudolph Contreras how he planned to plea in the packed federal courtroom.

A small group of protesters yelled "Lock him up!" as Flynn left the building, echoing the chant Flynn himself led against Trump's Democratic rival, Hillary Clinton, in his vitriolic appearances on the campaign trail.

Flynn's decision to take a plea deal and cooperate with the investigation led by Special Counsel Robert Mueller marked a major escalation in a probe that has dogged Trump's administration since the Republican president took office.

The White House said Flynn's guilty plea implicated him alone.

"Nothing about the guilty plea or the charge implicates anyone other than Mr. Flynn," Ty Cobb, a White House attorney, said in a statement.

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Prosecutors said that Flynn and Kislyak last December discussed economic sanctions that Washington had imposed on Moscow, and the U.N. Security Council vote regarded as damaging to Israel.

Flynn admitted later falsely telling FBI officials that he did not ask the ambassador to refrain from escalating a diplomatic dispute over the sanctions.

President Barack Obama's administration, which was still in office at the time, had imposed the sanctions for allegedly interfering in the election.

Flynn consulted with a senior member of Trump's presidential transition team about “what, if anything, to communicate to the Russian ambassador about the U.S. sanctions,” prosecutors said in a court document.

Flynn called a senior official of Trump’s transition team who was with other members of the team at Trump's Mar-a-Lago resort in Florida, the prosecutors said.

"Flynn called the Russian ambassador and requested that Russia not escalate the situation and only respond to the U.S. sanctions in a reciprocal manner,” the document said. It did not name the senior official in the Trump team.

On Dec. 28, 2016, the day before prosecutors say the call between the Trump aides took place, Trump publicly played down the need to sanction Russia for allegedly hacking U.S. Democratic operatives.

"I think we ought to get on with our lives. I think that computers have complicated lives very greatly," Trump told

reporters at Mar-a-Lago.

SCRUTINIZING TRUTHFULNESS OF TESTIMONY

Flynn, who faces up to five years in jail, was forced out of his White House post in February for misleading Vice President Mike Pence about his conversations with the Russian ambassador last December, after Trump's November election win and before the Jan. 20 inauguration.

The court document also said a "very senior official" in the Trump team told Flynn to contact foreign governments, including Russia, to try to influence their U.N. votes.

An official who worked with Trump's transition team and a source familiar with Mueller's investigation said Kushner was the official involved. Kushner's lawyer Abbe Lowell did not respond to multiple requests for comment about the issue. Lowell has previously said that Kushner has voluntarily cooperated with all relevant inquiries and would continue to do so.

Ryan Goodman, a professor at New York University Law School, said Flynn’s plea deal demonstrates that Mueller is closely scrutinizing the truthfulness of testimony given to his investigators. Kushner is potentially liable for making false statements if his testimony is contradicted by Flynn, Goodman said.

Both Flynn and Kushner also face potential liability under the Logan Act, a federal law which prohibits unauthorized U.S. citizens from negotiating with a foreign government, Goodman said.

But the Logan Act is very rarely enforced so it is unlikely Mueller would bring such a charge, Goodman said. “I don’t think Mueller is going to do anything radical,” Goodman said. “He is going to be conservative in the charges he brings.”

FLYNN AND TRUMP

Earlier on Friday, ABC News cited a confidant as saying Flynn was ready to testify that Trump directed him to make contact with Russians before he became president, initially as a way to work together to fight the Islamic State group in Syria.

Reuters could not immediately verify the ABC News report.

U.S. stocks, the dollar and Treasury yields fell sharply after the ABC report, although they partially rebounded after U.S. Senate Republicans said they had enough support to pass a tax overhaul bill later in the day.

If Trump directed Flynn to contact Russian officials, that might not necessarily amount to a crime. It would be a crime if it were proven that Trump directed Flynn to lie to the FBI about his contacts to the FBI.

Moscow has denied what U.S. intelligence agencies say was meddling in the election campaign to try to sway the vote in Trump's favor. Trump has called Mueller's probe a witch hunt.

In May, the president fired FBI Director James Comey, who later accused Trump of trying to hinder the agency's investigations into the Russia allegations. Comey also said he believed Trump had asked him to drop the FBI's probe into Flynn.

Comey on Friday tweeted a cryptic message about justice, but did not specifically mention the Flynn plea.

"But let justice roll down like waters and righteousness like an ever-flowing stream, 'Amos 5:24’," he wrote, quoting the Biblical book of Amos.

Flynn is the second former senior aide to Trump to be charged in the Mueller probe.

Paul Manafort, who ran Trump's presidential campaign for several months last year, was charged in October with conspiring to launder money, conspiracy against the United States and failing to register as a foreign agent of Ukraine’s former pro-Russian government.

Manafort, who did not join Trump's administration, and a business associate who was charged with him, pleaded not guilty.

(Additional reporting by Susan Heavey, Tim Ahmann, John Walcott, Mark Hosenball and Nathan Layne in Washington and Jan Wolfe in New York; Writing by Alistair Bell and Yara Bayoumy; Editing by Frances Kerry and Mary Milliken)