The Justice Department is dropping the criminal case against President Donald Trump's first national security adviser, Michael Flynn, whose lies about his contacts with Russia prompted Trump to fire him three years ago and special counsel Robert Mueller to flip him to cooperate in the Russia investigation.

The request to drop the case, filed with a federal judge in DC District Court on Thursday, is a sudden end to a protracted legal battle that's lately been fertile ground for Trump to attack the early Russia investigation and former FBI leadership he dislikes.

The court must still formally approve the request.In its filing, the department condemned the FBI's work when it interviewed Flynn in the West Wing in the first weeks of the Trump presidency.

The Justice Department called the FBI's counterintelligence investigation into Flynn for his contacts with Russia "a no longer justifiably predicated investigation," according to the filing.

"After a considered review of all the facts and circumstances of this case, including newly discovered and disclosed information appended to the defendant's supplemental pleadings, 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 that the FBI had, in the Bureau's own words, prepared to close because it had yielded an 'absence of any derogatory information.'"

The Justice Department also says it can't prove beyond a reasonable doubt that Flynn lied, nor that his lies were substantial.

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

Moreover, we not believe that the Government can prove either the relevant false statements or their materiality beyond a reasonable doubt," the filing states.On April 30, Trump suggested that Flynn would be exonerated and referred to the FBI agents as "dirty, filthy cops."

"It looks like to me that Michael Flynn would be exonerated based on everything that I see," Trump said. "I'm not the judge, but I have a different type of power."

Prosecutor quits

The Flynn filing was not signed by any career prosecutors at the Justice Department. Instead, the motion to dismiss was signed only by the politically appointed US Attorney Timothy Shea.

Jeff Jensen, the US attorney in St. Louis that Barr named earlier this year to review the handling of the Flynn case, said in a statement provided by the Justice Department that he recommended that the case be dismissed.

"Through the course of my review of General Flynn's case, I concluded the proper and just course was to dismiss the case. I briefed Attorney General Barr on my findings, advised him on these conclusions, and he agreed," Jensen said.

Minutes before the announcement, Brandon Van Grack, the lead Mueller prosecutor who cut the deal with Flynn, withdrew from it.

Van Grack gave no detail on the reason for his exit, writing only to the court to "please notice the withdrawal."