WASHINGTON — To his critics, Attorney General William Barr is doing all he can to undercut the conclusions of the Mueller investigation to please the White House. To his supporters, he is scrutinizing the probe of a presidential candidate that even the Justice Department’s inspector general said was flawed.

Those fault lines were in sharp relief after Barr directed federal prosecutors to abandon their prosecution of Michael Flynn, who served briefly as national security adviser in the early days of the Trump administration. Flynn admitted that he had lied to the FBI about his conversations during the transition with Russia’s ambassador to the U.S.

Sen. Richard Blumenthal, a Connecticut Democrat and member of the Senate Judiciary Committee, called Barr’s decision outrageous and said the justice system was being used for political ends. Rep. Jerry Nadler, the New York Democrat and chairman of the House Judiciary Committee, called for an investigation into Barr’s actions by the inspector general.

Many former prosecutors condemned the highly unusual move to drop the prosecution of someone who has twice entered a guilty plea in federal court.

Neal Katyal, who served in the Justice Department in the Obama administration, said it was “past stunning. “This is the collapse of the Justice Department as I and others who have worked there knew it.”

But Rep. Doug Collins, a Georgia Republican, said on Twitter, “After the government spent years destroying General Flynn’s life, justice has finally prevailed.” Brett Tolman, a former federal prosecutor in Utah, said: “This was the right decision, not a political decision. And the individuals who reviewed this were reviewing the evidence at issue absent any political mandate.”

Barr said he acted on a recommendation to drop the case by Jeffrey Jensen, a U.S. attorney in Missouri tasked by Barr in January to review the FBI’s investigation of Flynn.

“I concluded the proper and just course was to dismiss the case,” Jensen said Thursday. “I briefed Attorney General Barr on my findings, advised him on these conclusions, and he agreed.”

Flynn was charged with violating a federal law that makes it a crime to lie about any matter under investigation. But to be a crime, the lie must be about something “material,” something central to the investigation.

In its 20-page filing Thursday, the Justice Department said it concluded that Flynn’s lies were not material, for two reasons.

First, he had been under investigation since the summer of 2016 about whether he facilitated Russia’s meddling in the election. By early January, recently disclosed documents reveal, the FBI had concluded that he did not, and it was seeking to close the case. Then the FBI learned that Flynn was not telling the truth to Vice President Mike Pence and others in the White House about his conversations with the Russian ambassador.

Second, Thursday’s court filing said those conversations with the ambassador were not illegal. Therefore, it concluded, there was no investigative purpose in questioning Flynn, since agents had the transcripts of those calls. And even if he lied to the FBI, those lies weren’t material to any investigation.

“The reality is more mundane,” said the Lawfare website. “The FBI prepared to close an investigation and when presented with new evidence decided to keep it open. The fact that the investigation hadn’t been formally closed was a matter of bureaucratic convenience; the process of reopening a closed investigation requires paperwork and since this one hadn’t been closed, there was nothing to reopen.”

Barr’s decision to abandon the Flynn case renewed criticism that he was doing the president’s bidding.

He was accused in the spring of soft-pedaling the Mueller report before it was released. And in February, he instructed prosecutors to dial back their sentencing recommendation for former Trump campaign aide Roger Stone.

But Barr has been consistent in his view that the FBI’s investigation of the Trump campaign was overzealous. A year ago, he told Congress that he wanted to review “both the genesis and the conduct of intelligence activities directed at the Trump campaign.”

After the Justice Department inspector general concluded last December that the FBI’s investigation was not politically motivated, Barr said the FBI began looking at the Trump campaign based on the thinnest of suspicions and kept pushing even after it went nowhere.

“There has to be some basis before we use these very potent powers in our core First Amendment activity. And here, I felt this was very flimsy,” Barr said.

Thursday’s court filing does not end the Flynn case. The Justice Department has now asked Federal District Court Judge Emmet Sullivan to dismiss the prosecution and suggested in its court filing that the judge had little choice in the matter.

But Sullivan may want to look more deeply at the government’s version of what happened. “The judge may want to make some sort of inquiry to see if the dismissal is being sought for an appropriate purpose,” said Chuck Rosenberg, a former federal prosecutor.