Michael Flynn was charged with "willfully and knowingly" making "false, fictitious and fraudulent statements" regarding his meeting with the former Russian ambassador to the US, according to court documents. He has pleaded guilty, in what experts say signifies a major deal with prosecutors.
Mr Flynn is the first former Trump administration official to be charged in relation to special counsel Robert Mueller's Russia probe. Mr Mueller is investigating possible collusion with Russia and obstruction of justice by the Trump team. Three former Trump campaign aides have already been charged, and one has pleaded guilty.
Previous reports have claimed Mr Flynn spoke with then-Russian ambassador Sergey Kislyak last December, shortly after President Barack Obama imposed sanctions on Russia. According to current and former American officials, the incoming national security adviser promised that the situation would be different under Mr Trump.
Mr Flynn resigned in February, after it was revealed that he had misled the Vice President about these conversations.
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Now, prosecutors claim that Mr Flynn mislead the FBI as well. He allegedly told them that he did not ask Mr Kislyak to "refrain from escalating the situation in response to [the] sanctions". He also allegedly claimed not to remember Mr Kislyak telling him that Russia had "chosen to moderate its response to those sanctions as a result of his request".
The day after Mr Flynn and Mr Kislyak's conversation, Russian President Vladimir Putin declared he would not expel American diplomats from the country, despite the fact that Mr Obama had expelled 35 Russians from the US.
Donald Trump, then the US President-elect, praised the decision via Twitter.
“Great move on delay (by V. Putin),” he wrote. “I always knew he was very smart!”
Michael Flynn once said anyone seeking immunity 'probably committed a crime'
The charge against Mr Flynn is the latest sign that Mr Mueller's investigation is intensifying. Last month, the special counsel charged former Trump campaign manager Paul Manafort, and deputy Rick Gates, with various financial crimes. Both pleaded not guilty.
George Papadopolous, a former campaign adviser also charged by Mr Mueller, pleaded guilty to lying to the FBI about his contacts with Russia sources. Mr Papadopolous confessed to contacting a professor with Russian ties, who promised "dirt" on Hillary Clinton in the form of "thousands of emails". Mr Papadopoulos also repeatedly contacted other Trump campaign officials about arranging a meeting with the Russian government, according to court filings.
Comey: I took Trump's request to drop Flynn probe as a direction
Mr Flynn's lawyers have previously dismissed reports of his connections to the Russia investigation as "unfounded allegations, outrageous claims of treason, and vicious innuendo directed against him". But his legal team signalled they might be cooperating with the special counsel last week, when they ended an information-sharing deal with the White House.
Mr Trump, who has dismissed the entire Russia probe as a "witch hunt," has repeatedly come to Mr Flynn's defence. Shortly after the national security adviser stepped down, Mr Trump claimed he was simply "doing his job" by calling the Russian ambassador.
"I didn't direct him, but I would have directed him if he didn't do it," Mr Trump said at the time.
Former FBI Director James Comey also claimed that the President has pressured him to drop his investigation into Mr Flynn, telling him: "I hope you can see your way clear to letting this go, to letting Flynn go."
Mr Comey responded to the charges on Friday by tweeting a Bible verse about the "ever-flowing stream" of justice.
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