The department said in a filing Flynn's December 2017 guilty plea for lying to the FBI in an interview over his Russia contacts was moot because the lies were not significant.

It said the FBI's original probe of him had no "legitimate investigative basis."

The decision by the Justice Department, led by close Trump ally Attorney General Bill Barr, came as Flynn was fighting possible imprisonment, and after public statements by Trump that Flynn was the political victim of "filthy cops."

It sparked accusations that Barr was undermining longstanding department policies barring interference in cases involving political cronies.

"The evidence against General Flynn is overwhelming. He pleaded guilty to lying to investigators," said Jerry Nadler, the Democratic chairman of the House Judiciary Committee.

"And now a politicized and thoroughly corrupt Department of Justice is going to let the president's crony simply walk away."

Former FBI Deputy Director Andrew McCabe rejected the Justice Department's reasoning for dropping the case as "patently false" and "pure politics designed to please the president."

Flynn's secret talks with the Russian ambassador to Washington in December 2016, before Trump was inaugurated, was a cornerstone of the sprawling investigation by Special Counsel Robert Mueller into Moscow's meddling in the US election.

Even though Trump fired Flynn just 22 days into his administration, the president has always claimed the investigation was a political "witch hunt" and that Flynn, a former general and head of the Defense Intelligence Agency, was a "good man." 

The Justice Department filing Thursday gave support to Trump's claim, saying there were no grounds for the original investigation.

"The government has concluded that the interview of Mr Flynn was untethered to, and unjustified by, the FBI's counterintelligence investigation into Mr Flynn -- a no longer justifiably predicated investigation," it said.

 

Election year triumph for Trump 

The department said the January 24, 2017 interview was not "conducted with a legitimate investigative basis, and therefore does not believe Mr Flynn's statements were material if untrue."

Trump, who is campaigning for reelection in November, said Thursday after the decision that Flynn was "an innocent man."

"He was targeted by the Obama administration and he was targeted in order to try and take down a president, and what they've done is a disgrace," Trump said.

Trump took aim at the FBI and Justice Department officials who launched the probe of Flynn in August 2016, at the height of the presidential election battle that led to Trump's upset victory.

"I hope a lot of people are going to pay a big price, because they're dishonest crooked people. They're scum and I say it a lot. They're scum, they're human scum," he said.

 

Russian meddling 

Flynn, a senior Trump campaign advisor at the time, became one of the targets of an FBI investigation into Russian meddling in the election and efforts to support Trump.

The former Pentagon intelligence chief had a number of contacts with Russia and had been paid tens of thousands of dollars the previous year to attend a banquet in Moscow for a Russian media giant, where he sat next to President Vladimir Putin.

The investigation focused on Flynn's calls with Russian envoy Sergey Kislyak that December, after Trump's victory.

In them Flynn allegedly sought to make political deals for the incoming administration that were contrary to the positions of the outgoing Barack Obama administration.

Weeks after the FBI interview, Trump fired him for having lied to Vice President Michael Pence about his Russian contacts.

But the episode snowballed into an even broader investigation, after Trump then removed FBI director James Comey for rejecting his overtures to drop the Flynn probe.

In the wake of that, the Justice Department named Mueller as a special prosecutor to pursue the Russia investigation further infuriating Trump, who has repeatedly rejected the notion that Moscow interfered in the 2016 race on his behalf.

Flynn was one of six people associated with the Trump campaign who either pleaded guilty or were convicted in the investigation, which also issued indictments against 25 Russian individuals and three Russian companies.