US President Donald Trump has said the actions of former national security adviser Michael Flynn after the 2016 presidential election were lawful.
Mr Flynn has entered into a plea deal and agreed to co-operate with an inquiry into alleged collusion between Mr Trump's team and Russian officials.
The president said on Twitter that he fired Mr Flynn "because he lied to the Vice President and the FBI".
He said there was "nothing to hide" in the actions of his transition team.
Mr Flynn has pleaded guilty to making false statements to the FBI about contacts with Russia's then ambassador Sergei Kislyak in December 2016, after Mr Trump was elected but before he became president.
Under the terms of the plea deal - with Special Counsel Robert Mueller, who is investigating alleged collusion - Mr Flynn pleaded to a lesser charge and a shorter sentence than he might have otherwise faced.
It has prompted speculation that he has incriminating evidence on one or more senior members of the Trump administration.
The charging documents against Mr Flynn state that he was directed to make contact with Russian officials by a "very senior member" of the Trump transition team.
Several US news organisations report the very senior official now under the spotlight is Jared Kushner - Mr Trump's adviser and son-in-law.
US intelligence agencies say Russia's President Vladimir Putin directed a state effort to influence the US election in favour of Mr Trump.
Mr Trump has repeatedly denied that his campaign or transition team colluded in Russian actions. Speaking to reporters on Saturday, he said: "What has been shown is no collusion, no collusion. There has been absolutely no collusion. So we're very happy."
Mr Flynn was questioned by the FBI shortly after Mr Trump took office in January about his December meetings with the Russian ambassador. He was forced to resign in February, 23 days into his job.
According to the FBI's statement of offence signed by Mr Flynn, he discussed Russia's response to US sanctions as well as a UN Security Council resolution on Israel, at the direction of the Trump team.
Under the Logan Act it is illegal for a private US citizen, as Mr Flynn was during the transition period, to conduct foreign affairs without the permission or involvement of the US government.
The charge of making false statements normally carries up to five years in prison, but under the terms of his plea deal Mr Flynn faces a lighter sentence of only up to six months, court filings show.
The administration has sought to distance itself from Mr Flynn. White House lawyer Ty Cobb said on Friday: "Nothing about the guilty plea or the charge implicates anyone other than Mr Flynn."
Michael Flynn previously served in the Obama administration before being fired. Mr Obama reportedly advised his successor not to hire the former general, but Mr Trump appointed him to one of the most senior positions in the country.
Mr Flynn is not the first former Trump official to be charged. In October, Mr Trump's former campaign manager, Paul Manafort, and his business associate Rick Gates were accused of conspiring to defraud the US in dealings with Ukraine (both deny the charges).
Another ex-aide, George Papadopoulos, has also pleaded guilty to making false statements to FBI agents.
The revelation that Mr Flynn is cooperating with Mr Mueller overshadowed a significant victory for Mr Trump, whose sweeping tax reform bill passed the Senate on Friday night.
If signed into law, the bill, which scraped though with 51 votes to 49, would be the first significant legislative achievement of Mr Trump's presidency.