WASHINGTON — In an abrupt about-face, the Justice Department on Thursday said it is dropping the criminal case against President Donald Trump’s first national security adviser, Michael Flynn, abandoning a prosecution that became a rallying cry for the president and his supporters in attacking the FBI’s Trump-Russia investigation.

The action was a stunning reversal for one of the signature cases brought by special counsel Robert Mueller. It comes even though prosecutors for the past three years have maintained that Flynn lied to the FBI in a January 2017 interview about his conversations with the Russian ambassador.

Flynn himself admitted as much, pleading guilty before later asking to withdraw the plea, and he became a key cooperator for Mueller as the special counsel investigated ties between Russia and Trump’s 2016 political campaign.

Thursday’s action was swiftly embraced by Trump, who has relentlessly tweeted about the “outrageous” case and last week pronounced Flynn “exonerated,” and it is likely to energize supporters of the president who have taken up the retired Army lieutenant general as a cause.

But it will also add to Democratic complaints that Attorney General William Barr is excessively loyal to the president, and could be a distraction for a Justice Department that for months has sought to focus on crimes arising from the coronavirus.

“Attorney General Barr’s politicization of justice knows no bounds,” House Speaker Nancy Pelosi said. She accused Barr’s department of “dropping the case to continue to cover up for the president.”

Shortly before the filing was submitted, Brandon Van Grack, a Mueller team member and veteran prosecutor on the case, withdrew from the prosecution, a possible sign of disagreement with the decision.

After the Flynn announcement, Trump declared that his former aide had been “an innocent man” all along. He accused Obama administration officials of targeting Flynn and said, “I hope that a big price is going to be paid.” At one point he went further, saying of the effort investigating Flynn: “It’s treason. It’s treason.”

In court documents filed Thursday, the Justice Department said that after reviewing newly disclosed information and other materials, it agreed with Flynn’s lawyers that his interview with the FBI should never have taken place because his contacts with the Russian ambassador were “entirely appropriate.” The Flynn interview, the department said, was “conducted without any legitimate investigative basis.”

Barr Has Increasingly Challenged the Federal Trump-Russia Investigation

The U.S. attorney reviewing the Flynn case, Jeff Jensen, formally recommended dropping it to Barr last week, the course of action vehemently and publicly recommended by Trump, who appointed Barr to head the Justice Department.

Barr has increasingly challenged the federal Trump-Russia investigation, saying in a television interview last month that it was started “without any basis.” In February, he overruled a decision by prosecutors in the case of Roger Stone, a longtime Trump friend and adviser, in favor of a more lenient recommended sentence.

Jensen said in a statement that he “briefed Attorney General Barr on my findings, advised him on these conclusions, and he agreed.”

The department’s action comes amid an internal review into the handling of the case and an aggressive effort by Flynn’s lawyers to challenge the basis for the prosecution. The lawyers cited newly disclosed FBI emails and notes last week to allege that Flynn was improperly trapped into lying when agents interviewed him at the White House days after Trump’s inauguration.

None of the documents appeared to undercut the central allegation that Flynn had misled the FBI.

Thursday’s filing was the latest dramatic development in a years-old case full of twists and turns. In recent months, Flynn’s attorneys have leveled a series of allegations about the FBI’s actions and asked to withdraw his guilty plea. A judge has rejected most of the claims and not ruled on others, including the bid to revoke the plea.

Earlier this year, Barr appointed Jensen, the top federal prosecutor in St. Louis to investigate the handling of Flynn’s case.

As part of that process, the Justice Department gave Flynn’s attorneys internal FBI correspondence, including one handwritten note from a senior FBI official that mapped out internal deliberations about the purpose of the Flynn interview: “What’s our goal? Truth/admission or to get him to lie, so we can prosecute him or get him fired?” the official wrote.

Other documents show the FBI had been prepared weeks before its interview to drop its investigation into whether he was acting at the direction of Russia. 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 kept the investigation open to question him about that. Two agents visited him at the White House on Jan. 24, 2017.

Flynn Pleaded Guilty, Among the First of the President’s Aides to Admit Guilt

But Thursday’s filing, signed by District of Columbia U.S. Attorney Tim Shea, says the FBI had no basis to continue investigating Flynn after failing to find he had done anything illegal. It says there was nothing on his Russia calls “to indicate an inappropriate relationship between Mr. Flynn and a foreign power.” The department also contends Flynn’s answers during the interview were equivocal and indirect, rather than false, and weren’t relevant to the underlying investigation into whether the Trump campaign and Russia were illegally coordinating.

The memo also cites what it describes as internal uncertainty within the FBI over whether Flynn had lied, noting that the agents who interviewed him reported that he had a “very sure demeanor” and that-then FBI Director James Comey had said it was a “close” call.

In addition to Democratic lawmakers, former FBI Deputy Director Andrew McCabe lambasted the new action. He said the FBI was obligated to interview Flynn “to better understand why he was talking to Russian officials.” Flynn’s lies, McCabe said, “added to our concerns about his relationship with the Russian government.”

Flynn pleaded guilty, among the first of the president’s aides to admit guilt in Mueller’s investigation. He acknowledged that he lied about his conversations with Kislyak, in which he encouraged Russia not to escalate tensions with the U.S. over sanctions imposed by the Obama administration for election interference.

He provided such extensive cooperation that prosecutors said he was entitled to a sentence of probation instead of prison.

However, his sentencing hearing was abruptly cut short after Flynn, facing a stern rebuke from U.S. District Judge Emmet Sullivan, asked to be able to continue cooperating and earn credit toward a more lenient sentence.

His then-attorneys pointedly noted in their sentencing memo that the FBI had not warned him that it was against the law to lie when they interviewed him.

He later hired new attorneys, including conservative commentator Sidney Powell, who have taken a far more confrontational stance to the government. The lawyers accused prosecutors of withholding documents and evidence they said was favorable to the case and have repeatedly noted that one of the two agents who interviewed Flynn was fired for having sent derogatory text messages about Trump during the 2016 presidential campaign.