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.