The US Justice Department is dropping a case against a former national security adviser to President Donald Trump in relation to the investigation over Russia's alleged meddling in the 2016 US presidential election.
Trump, in reacting to the Justice Department's decision on Thursday, said he was very happy for Flynn.
In December 2017 Mr Flynn pleaded guilty to one count of lying to investigators, in a deal in which he avoided other charges including illegally lobbying for Turkey.
During an interview with CBS News' Catherine Herridge, Barr said the Justice Department had been investigating Flynn's accusation of misconduct by the government, and after finding additional material, he agreed the case should be dismissed.
In moving to drop Flynn's prosecution, the Justice Department stepped away from perhaps the highest-profile case brought by the special counsel's office, which officially ended its investigation one year ago.
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The Justice Department then named Mr Mueller as a special prosecutor to pursue the investigation into numerous contacts between Mr Trump's campaign and the Russians, which fuelled suspicions of collusion. Media reports have expressed concern over possible undermining of the independence of the Justice Department.
He provided such extensive cooperation that prosecutors said he was entitled to a sentence of probation instead of prison.
Flynn pleaded guilty that December, among the first of the president's aides to admit guilt in Mueller's investigation. I want Bill Barr to handle it. "What's our goal? Truth/admission or to get him to lie, so we can prosecute him or get him fired?"
"The evidence against General Flynn is overwhelming", Mr Nadler said. On the question of materiality, Barr responded, "we feel really that a crime can not be established here because because there was not, in our view, a legitimate investigation going on". Later that month, though, as the White House insisted that Flynn had never discussed sanctions with Russian Ambassador Sergey Kislyak, FBI officials grew more concerned by Flynn's conversations with the diplomat and what he had communicated to the White House.
Former FBI deputy director Andrew McCabe, who oversaw the Russian Federation investigation, said Mr Barr's argument is "patently false, and ignores the considerable national security risk his contacts raised". "And they asked him if he remembered saying that, and he said he didn't remember that".